FACIAL ACTION
CODING SYSTEM

– A Visual Guide

Explore the building blocks of facial expressions. This guide covers the core benefits, main Action Units (AUs), and how they combine to reveal human emotion.

Overview of FACS Action Units

The Facial Action Coding System (FACS) refers to a set of facial muscle movements that correspond to a displayed emotion. Originally created by Carl-Herman Hjortsjö with 23 facial motion units in 1970, it was subsequently developed further by Paul Ekman, and Wallace Friesen. The FACS as we know it today was first published in 1978, but was substantially updated in 2002.

Using FACS, we are able to determine the displayed emotion of a participant. This analysis of facial expressions is one of very few techniques available for assessing emotions in real-time. Other measures, such as interviews and psychometric tests, must be completed after a stimulus has been presented. This delay ultimately adds another barrier to measuring how a participant truly feels in direct response to a stimulus.

01. Three Core Benefits

// FACIAL_CODING

Non‑Intrusive

Captures emotional expressions in real time without verbal reporting – authentic and unbiased.

High Temporal Precision

Analyzes micro‑expressions and subtle changes in fractions of a second.

Objective & Quantifiable

Converts facial muscle movements into numerical data for rigorous analysis.

02. Main Action Units

// MUSCLE_MOVEMENT
AUVisualDescriptionFacial Muscle
1Inner Brow RaiserFrontalis, pars medialis
2Outer Brow RaiserFrontalis, pars lateralis
4Brow LowererDepressor Glabellae, Corrugator
5Upper Lid RaiserLevator palpebrae superioris
6Cheek RaiserOrbicularis oculi, pars orbitalis
7Lid TightenerOrbicularis oculi, pars palpebralis
9Nose WrinklerLevator labii superioris alaeque nasi
10Upper Lip RaiserLevator Labii Superioris
12Lip Corner PullerZygomatic Major
15Lip Corner DepressorDepressor anguli oris
20Lip StretcherRisorius
25Lips PartDepressor Labii, Orbicularis Oris relaxation
26Jaw DropMasetter, Temporal relaxed
ⓘ The full FACS manual defines over 100 Action Units. This table lists only the most common AUs directly involved in the seven universal emotions (happiness, sadness, surprise, fear, anger, disgust, contempt). AUs 27–50 omitted – these include rare or specialized movements (AU27 Mouth stretch, AU28 Lip suck, AU29–30 Jaw thrust/sideways, AU31–35 Cheek/jaw actions, AU36–40 Tongue movements, AU41–46 Eye closures/blinks/squints, AU47–50 Unassigned or rare head/eye combos). For most prompt engineering and applied research, focusing on AUs 1–26, 51–58, and 61–64 is sufficient.
$ AU combinations define discrete emotions. Happiness = AU6 + AU12 (Cheek Raiser + Lip Corner Puller).

03. Head Movement AUs

// ORIENTATION
AUMovement TypeDescriptionPreview
51Rotation (Yaw)Head Turn LeftHead Turn Left
52Rotation (Yaw)Head Turn RightHead Turn Right
53Rotation (Pitch)Head UpHead Up
54Rotation (Pitch)Head DownHead Down
55Rotation (Roll)Head Tilt LeftHead Tilt Left
56Rotation (Roll)Head Tilt RightHead Tilt Right
57TranslationHead ForwardHead Forward
58TranslationHead BackHead Back
ⓘ This is the complete set of head movement AUs (AU51–58) defined in FACS. No AUs are omitted here. (AUs 59–60 are not assigned in the standard FACS system.)

04. Eye Movement AUs

// GAZE_VECTOR
AUMovement TypeDescriptionPreview
61Horizontal GazeEyes Turn LeftEyes Turn Left
62Horizontal GazeEyes Turn RightEyes Turn Right
63Vertical GazeEyes UpEyes Up
64Vertical GazeEyes DownEyes Down
ⓘ This is the complete set of eye movement AUs (AU61–64) defined in FACS. No AUs are omitted here.

05. Emotions & Action Units

// COMBINATION_LOGIC
EmotionAUsDescriptionPreview
Happiness6 + 12Cheek Raiser, Lip Corner PullerHappiness
Sadness1 + 4 + 15Inner Brow Raiser, Brow Lowerer, Lip Corner DepressorSadness
Surprise1 + 2 + 5 + 26Inner/Outer Brow Raiser, Upper Lid Raiser, Jaw DropSurprise
Fear1+2+4+5+7+20+26Brows raised + lowered, lid tightener, lip stretcher, jaw dropFear
Anger4 + 5 + 7 + 23Brow Lowerer, Upper Lid Raiser, Lid Tightener, Lip TightenerAnger
Disgust9 + 15 + 16Nose Wrinkler, Lip Corner Depressor, Lower Lip DepressorDisgust
Contempt12 + 14 (unilateral)Lip Corner Puller + Dimpler (one side of face)Contempt
ⓘ These seven emotions represent the universal facial expressions identified by Ekman. Other emotional states (e.g., embarrassment, pride, pain) involve more complex or subtle AU combinations not covered here.
Use these AU combinations in video prompts to lock emotional intent: "Character shows AU6+AU12 – genuine smile with eye constriction."

FACS prompt pattern

When generating AI video, anchor facial expressions with explicit AUs:

$"Female, mid 30s, neutral lighting. Display AU1+AU2+AU5+AU26 (surprise). Eyes wide, brows raised, jaw slightly dropped. Zero smile."

This prevents the model from defaulting to neutral or mixed expressions.