A complete plain-English decode of every Facial Action Unit in the Facial Action Coding System — what each one does, the muscle that drives it, and the exact prompt phrasing for AI image and video generation. Why your AI portraits look dead, and the 1978 cheat sheet that fixes them.
Artifio Build Team22 min read6,200 words
01
Why AI faces default to lifeless smiles — and how to fix it
02
Every Action Unit decoded, with the muscle it drives
03
The seven emotion recipes, ready to drop into a prompt
04
Pro tips on stacking, intensity, and the asymmetry tell
Why this exists
§1
If you type “a smiling woman, happy” into any frontier image model, you get the same dead-eyed AI smile. Mouth turned up. Eyes blank. The face reads like a mannequin. Now type “a woman, cheeks lifted, lip corners pulled, eyes crinkled with crow’s feet, soft daylight” and the same model produces a face you’d believe.
The difference isn’t model quality. It’s vocabulary. Modern image and video models have absorbed millions of photographs annotated with anatomical descriptors. They respond to muscles, not moods. Naming the muscle is the prompt-engineering unlock.
The vocabulary is already published. It’s called the Facial Action Coding System — a 1978 framework from psychologists Paul Ekman and Wallace Friesen that catalogued every visually distinguishable facial movement into about thirty numbered Action Units. Each unit is a single muscle (or coordinated group). Stack a few of them and you have an emotion. This guide gives you the whole vocabulary, in plain English, with the prompt phrasing already written.
How FACS actually works
§2
FACS doesn’t say “happy.” It says AU 6 + AU 12 — cheek raiser plus lip-corner puller. Stack those two and you have a Duchenne smile. Drop AU 6 and you have the polite-photo smile that looks faintly dead. The system is mechanical, exhaustive, and exact, and that’s exactly what makes it useful as a prompting layer.
There are three families of Action Units. Main AUs describe facial muscle moves — brows, cheeks, lips. Head movement AUs describe gross head pose. Eye gaze AUs describe where the eyes are pointed. A complete expression specification is some combination of all three.
Below, each AU is decoded the same way: number and name, the muscle (Latin name, for accuracy), what it physically looks like, and a copy-pasteable prompt fragment. Mix and match.
How to read each entry
The number and name come from FACS proper. The muscle name is for your own reference — modern image models don’t need it, but knowing the anatomy helps you visualize what you’re asking for. The prompt fragment is what you actually paste.
Brows & upper eyes
AU 1 — 7
The brow region carries roughly 80% of the perceived emotion in a face. Get the brows right and the rest forgives a lot. Get them wrong and no smile in the world saves you.
AU 1 · Inner Brow Raiser
The worried brow.
Frontalis, pars medialis
Only the inner ends of the eyebrows lift toward the hairline — the outer ends stay flat or drop. Creates a slight upward V at the centre of the forehead. Reads as concern, sympathy, or quiet sadness.
Promptinner brows raised slightly, outer brows neutral, small forehead crease above the nose
AU 2 · Outer Brow Raiser
The arched brow.
Frontalis, pars lateralis
The outer ends of the brows arch upward while the inner ends stay put. Common in surprise and skeptical curiosity. When only one side fires (unilateral), it reads as cool detachment.
Promptouter brow arched upward, inner brow steady — or “one brow raised” for the skeptical look
AU 4 · Brow Lowerer
The furrow.
Corrugator supercilii, depressor supercilii
Brows pull down and toward each other, creating vertical lines between them. The default brow of concentration, anger, confusion, and concern. Stronger versions wrinkle the skin between the brows visibly.
Promptbrows drawn down and together, vertical crease between brows, slight tension above the eyes
AU 5 · Upper Lid Raiser
Wide-eyed.
Levator palpebrae superioris
The upper eyelid pulls up, revealing more of the iris and a slice of sclera above it. Reads as alertness, surprise, or threat. With AU 4 it becomes glaring or alarmed; with AU 26 it becomes shocked.
Promptupper eyelid raised, eyes opened wider than neutral, more white of the eye visible above the iris
AU 6 · Cheek Raiser
The eye crinkle.
Orbicularis oculi, pars orbitalis
The signature of a real smile. The cheeks lift, the lower eyelid pushes up slightly, and crow's-feet wrinkles fan out from the outer corners of the eyes. Without this AU, AU 12's lip pull reads as a fake or polite smile.
The lower eyelid tightens upward without cheek lift. The eye looks narrower and more focused. Common in anger, suspicion, and intense concentration. Adds menace to AU 4 + 5.
Promptlids tightened, eyes narrowed without smiling, focused stare
Nose & mid-face
AU 9 — 14
The middle of the face does most of the work for disgust, sneers, and the subtler smile variants. These AUs are small movements with big perceptual weight.
AU 9 · Nose Wrinkler
The disgust nose.
Levator labii superioris alaeque nasi
Skin on the bridge of the nose wrinkles upward. The upper lip is pulled up at the same time. This is the universal “something smells bad” face — the core marker of disgust.
Promptnose wrinkled, bridge of the nose creased, upper lip pulled slightly up
AU 10 · Upper Lip Raiser
The sneer.
Levator labii superioris
Upper lip pulls up, revealing more upper teeth and lifting the nasolabial fold. Reads as a sneer in isolation, or as a strong expressive smile when paired with AU 12.
Deepens the diagonal line from the side of the nose to the corner of the mouth, without other major lip movement. Adds maturity, weariness, or quiet seriousness.
The classic smile motor. The corners of the mouth pull up and outward. On its own it's a polite smile; with AU 6 it becomes a Duchenne smile; with AU 10 it widens into a beam.
Promptlip corners pulled up and outward — pair with AU 6 for genuine joy
AU 13 · Sharp Lip Puller
Vertical smirk.
Levator anguli oris (caninus)
Pulls the corners of the mouth straight up, more vertical than AU 12. Produces a sharper, more compressed smile. Often read as cocky or sly.
Promptcorners of mouth pulled sharply upward, vertical pull
AU 14 · Dimpler
Dimples in.
Buccinator
Creates small dimples or indentations at the corners of the mouth without lifting the lips. The signature of a half-smirk, suppressed smile, or — paired with one-sided AU 12 — contempt.
Promptsmall dimples at the corners of the mouth, lips otherwise still
Mouth & lips
AU 15 — 28
The mouth is where most prompts go wrong — saying “open mouth” gives you a cartoon O. The fixes are below.
AU 15 · Lip Corner Depressor
The frown.
Depressor anguli oris (triangularis)
Corners of the mouth pull downward. The core marker of sadness, disappointment, or disapproval. Subtle is more powerful than caricatured.
Promptlip corners drawn down, slight downturn at the mouth
AU 16 · Lower Lip Depressor
Lower teeth visible.
Depressor labii inferioris
Pulls the lower lip downward, exposing the lower teeth. Common in disgust, exertion, and forceful speech. Almost always co-occurs with AU 25.
Promptlower lip pulled down, lower teeth visible
AU 17 · Chin Raiser
The pushed-up chin.
Mentalis
The chin muscle contracts upward, pushing the lower lip up and wrinkling the skin of the chin. Reads as stubbornness, restrained emotion, or pouting.
The jaw drops passively — mouth opens but in a soft, slack way. The single best prompt fragment for convincing surprise. Cartoon-O mouths come from over-prompting; “slack jaw” stays human.
Promptjaw slack and slightly dropped, mouth open in a relaxed way
AU 27 · Mouth Stretch
Full open.
Pterygoids, digastric
An active wide opening of the mouth — yawning, screaming, gasping. Use sparingly; AU 26 carries surprise better in most contexts.
Promptmouth opened wide, jaw pulled down forcefully
AU 28 · Lip Suck
Sucked in.
Orbicularis oris
Lips pulled inward and pressed between the teeth. Suppressing a reaction, nervous habit, holding back tears.
Promptlips sucked inward, partially hidden between the teeth
Jaw, cheeks, tongue & nose
AU 8 — 39
Beyond the core set, FACS catalogues the lower-face action descriptors — jaw, cheek, tongue, lip, and nostril movements — plus a few less-common units. They're gold for the expressions emotion labels never name: a clenched jaw, a bitten lip, flared nostrils, a tongue pushed into the cheek.
AU 8 · Lips Toward Each Other
Lips drawn in.
Orbicularis oris
Both lips move toward each other and slightly inward, narrowing the mouth before speech or a pucker. Subtle — usually a transitional or anticipatory action rather than an expression on its own.
Promptlips drawn toward each other, mouth slightly narrowed, on the verge of speaking
AU 19 · Tongue Show
Tongue out.
Genioglossus
The tongue is visible between the lips or teeth — stuck out, or resting on the lower lip. Reads as concentration, cheekiness, distaste, or play depending on the rest of the face.
Prompttip of the tongue showing between the lips
AU 21 · Neck Tightener
Corded neck.
Platysma
The platysma tenses, pulling the skin of the neck taut so the vertical neck cords stand out. Signals strain, effort, fear, or rage held in the body.
Promptneck muscles tensed, vertical cords standing out on the neck, jaw set
AU 29 · Jaw Thrust
Jaw forward.
The lower jaw juts forward past its resting line. Reads as defiance, aggression, or stubborn determination.
Promptlower jaw thrust forward, slight underbite to the set of the mouth
AU 30 · Jaw Sideways
Jaw shifted.
The lower jaw slides to one side. Often a thinking, chewing, or skeptical gesture.
Promptjaw shifted to one side, mouth slightly askew
AU 31 · Jaw Clencher
Clenched jaw.
Masseter
The jaw muscles clench, the masseter bulges at the back of the jaw, and the jawline tightens. The classic restrained-anger or bracing-for-impact tell.
Promptjaw clenched, masseter flexed at the back of the jaw, tight jawline
AU 32 · Lip Bite
Biting the lip.
The teeth press into or bite a lip — usually the lower one. Reads as nervousness, anticipation, concentration, or flirtation.
Promptlower lip caught lightly between the teeth
AU 33 · Cheek Blow
Blowing out.
The cheeks fill and air is pushed out through pursed lips. Exhaustion, exasperation, or relief — the held breath finally let go.
Promptcheeks filled with air, blowing out through pursed lips
AU 34 · Cheek Puff
Puffed cheeks.
Both cheeks puff out with trapped air, lips sealed. Holding the breath, comic effort, or a reaction held in.
Promptcheeks puffed out with trapped air, lips pressed shut
AU 35 · Cheek Suck
Hollowed cheeks.
The cheeks are sucked inward against the teeth, hollowing the face — the fish-face. Reads as wry, considering, or deliberately posed.
Promptcheeks sucked inward, hollowed beneath the cheekbones
AU 36 · Tongue Bulge
Tongue in cheek.
The tongue pushes against the inside of a cheek or lip, raising a visible bulge. Literally tongue-in-cheek — irony, a suppressed reaction, or probing a tooth.
Prompttongue pushed into the cheek from the inside, a visible bulge in the cheek
AU 37 · Lip Wipe
Licking the lips.
The tongue moves across the lips. Anticipation, hunger, dryness, or nerves.
Prompttongue moving across the lips, caught mid-lick
AU 38 · Nostril Dilator
Flared nostrils.
Nasalis, pars alaris
The nostrils widen and flare, opening the nasal passages. Strong feeling — anger, exertion, arousal — or a sharp inhale.
Promptnostrils flared wide, openings of the nose widened
AU 39 · Nostril Compressor
Pinched nostrils.
Nasalis, pars transversa
The nostrils narrow and compress, pinching the sides of the nose. Distaste, restraint, or a held breath.
Promptnostrils compressed and narrowed, sides of the nose pinched in
Eyelid & blink
AU 41 — 46
For video and animation prompts: eyelid behaviour is what separates a frozen-doll loop from believable motion.
AU 41 · Lid Droop
Heavy lids.
Relaxation of levator palpebrae superioris
Upper lids hang lower than neutral. Reads as drowsy, intoxicated, or sultry depending on context.
Promptupper eyelids drooped, heavy lidded look
AU 42 · Slit
Slit eyes.
Orbicularis oculi
Eyes narrow into a thin horizontal slit without the squint of AU 44. Reads as cold, suspicious, or sleepy.
Prompteyes narrowed into a horizontal slit, cold expression
AU 43 · Eyes Closed
Closed.
Levator palpebrae fully relaxed
Eyes fully closed, lids relaxed. Sleep, peace, a held breath.
Prompteyes closed, lids relaxed
AU 44 · Squint
Active squint.
Orbicularis oculi, pars palpebralis
Both lids actively tightened together — the bright-sun squint or the looking-hard-at-something squint. Different from AU 42's passive slit.
Prompteyes squinted tightly against bright light, lids actively pulled in
A normal involuntary blink. For video models, asking for a blink mid-shot prevents the dead-stare effect and adds liveness for free.
Promptsingle natural blink mid-shot
AU 46 · Wink
Wink.
Levator palpebrae + orbicularis oculi, one side only
One eye closes deliberately while the other stays open. Conspiratorial, flirtatious, or playful depending on the rest of the face.
Promptone eye winked, the other eye open, slight smile
Head movement
AU 51 — 58
Head pose AUs read as direct commands by most image models. Useful for setting camera-subject relationships without leaving prompt language.
AU 51 · Head Turn Left
Look left.
Head rotates to the subject's left around the vertical axis. Three-quarter angles from the camera's perspective.
Prompthead turned to her left, three-quarter view
AU 52 · Head Turn Right
Look right.
Mirror of AU 51. Head rotates to the subject's right.
Prompthead turned to his right, three-quarter angle
AU 53 · Head Up
Chin lifted.
Head tilts back, chin rises. Reads as superior, defiant, or contemplative.
Promptchin lifted, head tilted slightly back
AU 54 · Head Down
Chin tucked.
Head drops forward, chin tucks toward the chest. Submission, shyness, or thought.
Promptchin tucked toward chest, head down, eyes looking up under brows
AU 55 · Head Tilt Left
Tilt left.
Head tilts sideways toward the subject's left shoulder. Curious, listening, soft.
Prompthead tilted gently to her left, ear toward shoulder
AU 56 · Head Tilt Right
Tilt right.
Mirror of AU 55.
Prompthead tilted gently to his right
AU 57 · Head Forward
Leaning in.
Whole head pushes forward toward the camera. Engaged, intent, predatory.
Prompthead pushed forward toward the camera, neck extended
AU 58 · Head Back
Pulled back.
Head pulls back away from the camera. Recoil, hesitation, shock.
Prompthead pulled back from the camera, slight recoil
Eye gaze
AU 61 — 64
Gaze direction shifts perceived emotion drastically with no other change. Downcast eyes read as sadness or modesty even on a neutral face; upward gaze reads as awe or reflection.
AU 61 · Eyes Turn Left
Glance left.
Eyes look to the subject's left without head movement.
Prompteyes glanced to her left, head facing camera
AU 62 · Eyes Turn Right
Glance right.
Eyes look to the subject's right without head movement.
Prompteyes glanced to his right, head facing camera
FACS treats emotions as recipes. Each universal emotion is a specific stack of Action Units. The recipes below are the standard ones — Ekman’s originals, lightly modernized for prompt phrasing.
Happiness / Joy
AU 6 + 12
Cheek raiser plus lip-corner puller. This is the Duchenne smile — the only smile that reads as genuine. Drop AU 6 and you get the polite-photo grin. Everything else about a “happy” prompt is decoration on these two muscles.
Promptwoman, cheeks lifted, lip corners pulled up and outward, eyes crinkled with crow's feet, light squint at the lower lids, soft daylight
Sadness
AU 1 + 4 + 15
Inner brow raiser, brow lowerer, lip corner depressor. The classic sad face — inner brows pulled up while the rest pulls down. A small amount of AU 17 (chin raiser) can add the trembling-lip suppression layer.
Promptman, inner brows pulled up and together, outer brows lowered, lip corners drawn down, eyes downcast, soft natural light
Surprise
AU 1 + 2 + 5 + 26
Both brows raised, upper lid raised, jaw drops. The jaw is the giveaway — passively slack, not actively open. Pure surprise is brief; if you want it to read on a still frame, ask for a “caught mid-reaction” quality.
Promptwoman caught mid-reaction, both brows raised high, upper eyelids lifted, jaw slack and slightly dropped, eyes wide on the camera
Fear
AU 1 + 2 + 4 + 5 + 7 + 20 + 26
The busiest face in the catalogue. Brows raised and drawn together at the same time (the rarest brow configuration), upper lids high, lower lids tight, mouth stretched horizontally, jaw open. Stack at least five of these or fear collapses into surprise.
Promptman, brows raised AND drawn together, upper lids high, lower lids tight, lips stretched horizontally toward the ears, jaw dropped open, head pulled back slightly
Anger
AU 4 + 5 + 7 + 23
Brow lowered, upper lid raised, lid tightener, lip tightener. Anger reads angriest when still — the face is tense rather than animated. A snarl loses menace; a frozen glare keeps it.
Promptwoman, brow lowered and drawn together, upper lid raised, lids tight, lips pressed thin, jaw set, direct stare into camera, soft side light
Disgust
AU 9 + 15 + 16
Nose wrinkler, lip corner depressor, lower lip depressor. The nose is doing most of the work — without AU 9, this face just reads as disappointed. The lower lip exposes lower teeth as part of the same recoil reflex.
Promptchild, nose wrinkled with skin creased on the bridge, upper lip pulled up slightly, lip corners drawn down, lower lip pulled down showing lower teeth
Contempt
AU 12 + 14 (one side only)
The only universally recognized asymmetric emotion. Lip corner puller plus dimpler — but only on one side. The other half of the face stays neutral. The asymmetry is the tell; if both sides fire it just reads as a smirk.
Promptman, left lip corner pulled up slightly with a small dimple, right side of the face neutral, eyes steady, slight head tilt
Pro tips
§ Combinations & intensity
Six meta-patterns that take FACS prompting from “correct” to genuinely good.
Stack thoughtfully
Three AUs is the floor.
A single AU is a tic, not an emotion. Two AUs is a hint. Three or more makes the model commit to the expression. For complex emotions like fear or grief, push toward five.
Asymmetry is the tell
One side, not both.
Real human faces fire asymmetrically. Contempt is the textbook case (AU 12 + 14 unilateral), but adding “left side stronger than right” or “one brow raised” to any expression makes it read as a candid moment rather than a posed one.
Intensity words
Subtle > obvious.
FACS uses A through E for intensity. In prompt terms: words like “slightly,” “just barely,” “the beginning of,” and “trace of” produce more believable faces than “fully,” “maximum,” or “exaggerated.” Models over-render anything you push to 11.
Don't name the emotion
Describe, don't label.
The fastest way to a dead AI face is the word “happy” or “angry” in the prompt. The model already has those concepts compressed into clichés. Describe the muscles instead and you get a face that hasn't been seen before.
Pair with light
Soft light forgives.
Soft, directional light (window light, golden hour) sells facial expressions because shadows resolve the muscle structure for the model. Flat overhead light flattens everything you just specified.
Add a verb of motion
“Caught mid-X” works.
For stills, phrases like “caught mid-laugh,” “mid-recoil,” “in the middle of an exhale” produce more dynamic faces than the static emotion alone. For video, this gives the model a clear motion anchor.
Glossary
§ Reference
Key terms used throughout this guide.
Action Unit (AU)
A single visually distinguishable facial muscle movement, numbered in the Facial Action Coding System. Each AU corresponds to one muscle or coordinated group.
Asymmetry
When an AU fires on only one side of the face. Contempt (AU 12 + 14) is the only universally asymmetric emotion, but adding asymmetry to any expression increases candidness.
Duchenne smile
A genuine smile combining AU 6 (cheek raiser) and AU 12 (lip corner puller). Named for French neurologist Guillaume Duchenne. Distinguished from the polite smile by the involvement of the eye orbicularis muscles.
FACS
The Facial Action Coding System. A 1978 framework by Paul Ekman and Wallace Friesen that catalogues every visually distinguishable facial muscle movement into numbered Action Units.
Intensity
FACS rates each AU's strength on a five-point A–E scale. In prompt terms: words like “slightly”, “trace of”, or “the beginning of” produce more believable faces than max-intensity descriptors.
Polite smile
A smile produced by AU 12 alone, without AU 6. Mouth turns up, eyes do not. This is the default AI face when prompted with “smiling”.
Recipe
A specific combination of Action Units that produces a recognizable emotion. The seven universal emotion recipes are listed in this guide.
Stack
To combine multiple AUs in one prompt. Three AUs is the floor for a model to commit to an expression; complex emotions like fear require five or more.
Frequently asked questions
§ FAQ
The questions that come up most often when using FACS as a prompt-engineering reference.
What is the Facial Action Coding System (FACS)?
FACS is a 1978 framework, developed by psychologists Paul Ekman and Wallace Friesen, that catalogues every visually distinguishable human facial movement into about thirty numbered units called Action Units (AUs). Each AU corresponds to a single muscle or coordinated muscle group. Specific combinations of AUs map to the seven universal emotions.
Who created FACS?
FACS was developed by psychologists Paul Ekman and Wallace V. Friesen and first published in 1978. It built on earlier work by Carl-Herman Hjortsjö from 1970 and was substantially updated in 2002.
How many Action Units are there in FACS?
FACS defines roughly 30 main Action Units for facial muscles, plus lower-face action descriptors (jaw, cheek, tongue, nose), head-movement AUs (51–58), and eye-gaze AUs (61–64) — well over fifty codes in all. This guide documents 56, the set most relevant to AI image and video prompting.
What are the seven universal emotions in FACS?
Happiness (AU 6+12), sadness (AU 1+4+15), surprise (AU 1+2+5+26), fear (AU 1+2+4+5+7+20+26), anger (AU 4+5+7+23), disgust (AU 9+15+16), and contempt (AU 12+14 on one side of the face only).
How do I use FACS to write better AI image prompts?
Replace emotion labels ('happy', 'angry') with the physical muscle movements that produce them ('cheeks lifted, lip corners pulled, eyes crinkled'). Modern image and video models respond more accurately to anatomical descriptions than to emotion words, because their training data includes millions of annotated face photographs.
What is a Duchenne smile in FACS terms?
A Duchenne smile combines AU 12 (lip corner puller) with AU 6 (cheek raiser). The AU 6 component produces the crow's-feet wrinkles around the eyes that distinguish a genuine smile from a polite-photo smile, which uses AU 12 alone.
Why does my AI face look dead even with a good prompt?
Most often because the prompt asks for an emotion ('smiling, happy') instead of describing the muscles. Without the muscle vocabulary, the model defaults to the most common photographed version of that emotion — the polite-stock-photo smile that reads as dead. FACS-informed prompts that name the cheek raiser, the lip corner puller, and the crinkled eyes consistently produce more believable faces.
Which AI models respond best to FACS-style prompts?
Models with strong face fidelity in 2026 respond best: Veo 3.1, Grok Imagine Video 1.5, Nano Banana 2, and Seedance 2 are reliable. All available through Artifio, pay-as-you-go, no subscription.
Is FACS the same as facial recognition?
No. Facial recognition identifies who a face belongs to. FACS describes what a face is doing — which muscles are moving. They are different problems and use different techniques.
Can I use FACS prompts in video generation?
Yes, and video benefits even more. Adding eyelid behaviour (AU 45 blink, AU 41 lid droop) and head motion (AU 51–58) to a video prompt prevents the frozen-doll loop that plagues otherwise-good face videos.
Do I need to memorize muscle names like 'zygomaticus major'?
No. Modern image models respond to plain descriptions ('lip corners pulled up and outward'). The Latin muscle names are useful for your own understanding of what you're asking for, but the prompt itself works in everyday English.
What is the most common mistake in prompting faces?
Asking for an exaggerated expression ('maximum joy', 'extremely angry'). Models over-render anything pushed to maximum. Subtler intensity words — 'slightly', 'the beginning of', 'trace of' — consistently produce more believable faces.
Try any of the prompts above on 100+ frontier models — Grok Imagine Video 1.5, Veo 3.1, Nano Banana 2, Seedance 2, and the rest — all in one workspace. Pay as you go. No subscription. $1 to start.
We build tooling for AI image and video generation at Artifio.ai. This guide is part of our ongoing prompt-engineering reference series. Last updated .