Chapter 3: Expression Sheets & Appeal

Created by Sarah Choi (prompt writer using ChatGPT)

Expression Sheets (FACS‑Informed) & Appeal

For character concept artists across concepting and production. Focus: craniofacial structure, features, and emotions.

1) Why Expression Sheets Matter

Expression sheets are the truth‑tables of a character’s face. They demonstrate how craniofacial structure deforms across emotions, prove that features were designed with motion logic, and give downstream teams a reference to sculpt blendshapes, wire facial rigs, and tune shader responses. A good sheet balances anatomical credibility (FACS‑informed) with appeal (graphic clarity, charm, and character intent). The goal is consistency across angles and lighting, with readability at three distances: thumbnail, gameplay camera, and close‑up.

2) FACS in Production—A Practical Lens

The Facial Action Coding System (FACS) labels observable muscle actions (Action Units, AUs). You don’t need to memorize the entire catalog to design expressive packages. Use a production subset mapped to common story beats:

  • Brow Actions: AU1 (inner brow raise), AU2 (outer brow raise), AU4 (brow lowerer). These rotate the forehead/brow planes and create distinctive glabella rhythms.
  • Eye/Lid Actions: AU5 (upper lid raise), AU6 (cheek raise), AU7 (lid tighten). These alter the eyelid shelves and infraorbital bevels.
  • Nose Actions: AU9 (nose wrinkle), AU10 (upper lip raise). These sculpt the alar wings and nasal under‑plane.
  • Mouth/Lower Face: AU12 (lip corner puller), AU14 (dimpler), AU15 (corner depressor), AU17 (chin raiser), AU20 (lip stretcher), AU23/24 (lip tightener/pressor), AU25/26/27 (part/jaw drop/stretch). These rotate the dental cylinder, corners, and chin pad.

Treat AUs as building blocks. Expressions are recipes—e.g., Duchenne smile ≈ AU6 + AU12; anger ≈ AU4 + AU5/7 + AU23 (+ AU24), sadness ≈ AU1 + AU15, disgust ≈ AU9 + AU10 (often asymmetric), fear ≈ AU1 + AU2 + AU5 + AU20/25.

3) Craniofacial Mechanics: What Physically Moves

Appeal improves when deformation follows believable mechanics. Brows pivot around the supraorbital rim; eyelids slide over a spherical eyeball; alar wings flare along soft cartilage; lip corners hinge along the commissure and travel on the dental cylinder; the mentalis lifts the chin pad; the jaw rotates and translates at the TMJ. Honor these axes. In stylized designs, you may exaggerate the amplitude, but keep pivot logic intact to preserve credibility.

4) The Expression Sheet: Layout & Coverage

Design your sheet as a matrix that proves both range and control.

Core set (per character): Neutral; Joy (Duchenne: AU6+12); Anger (AU4+5/7+23/24); Sadness (AU1+15); Fear/Surprise (AU1+2+5+25); Disgust (AU9+10, often 1‑sided). Each should be shown front and ¾ to validate plane rotations. Add asymmetry variants for smirks, sneers, and skeptical brows, and phoneme extremes (A, E, F/V, M, L, W‑OO) on a separate row.

Angles: At minimum, front + ¾. For hero characters, include profile for nose/upper‑lip/chin reads. Keep lighting consistent per row so changes read as deformation, not relighting.

Annotations: Under each tile, list dominant AUs, plane notes (e.g., “lower lid shelf compressed”), and rig flags (e.g., “requires corner forward slide”). Use arrows sparingly to indicate direction of motion for corners, brows, and alar wings.

5) Appeal: The Three‑Tier Read

Appeal is a designed value hierarchy and shape rhythm that survives simplification.

  1. Silhouette tier: brow contour, eyelash arc, mouth corner trajectory, jaw line. At thumbnail, the outer shapes must already communicate mood.
  2. Mid‑plane tier: forehead steps, zygoma/cheek lift, infraorbital shelf, nasal under‑plane, lip cylinder. These carry the bulk of emotional nuance.
  3. Micro tier: wrinkles, pores, and secondary creases. Only add after tiers 1–2 read; otherwise you create noise without clarity.

Design rhymes across features (e.g., a V‑shaped brow rhythm echoes a sharp cupid’s bow in anger; soft arcs echo in joy). Keep asymmetry—faces are rarely perfectly mirrored, and slight offsets increase charm.

6) Readability Across Styles

  • Realistic: Smaller amplitude, more reliance on plane transitions and wrinkle maps. Prioritize anatomical pivot points and material separation (wet line, tear meniscus).
  • Semi‑real: Exaggerate arc lengths and compression; simplify wrinkle patterns into designed folds.
  • Stylized/Graphic: Trust silhouette and two‑to‑three value steps. Use a single bold specular in eyes; indicate under‑plane of nose and lower‑lip shelf as graphic shapes.

Whatever the style, corner architecture of the mouth and lid thickness are non‑negotiable for believable motion.

7) Building Each Emotion: Practical Recipes

Joy (Duchenne): AU6 lifts cheek mass; AU12 pulls corners up/back on the cylinder; upper lid narrows; crow’s‑feet bevels form. Teeth exposure increases—design a dental show strategy (which teeth, how much, and when).

Anger: AU4 drags brows down/in—glabella becomes a sharp V; AU5/7 tighten lids; AU23/24 press lips—upper lip thins, nasolabial fold deepens. Jaw may protrude slightly.

Sadness: AU1 raises inner brows into a tilted roof; AU15 pulls corners down; lower lid lengthens; upper lip relaxes. The philtrum softens; chin pad may lift (AU17) creating a pout.

Fear/Surprise: AU1+2 elevate inner/outer brows; AU5 opens upper lids showing sclera superiorly; AU25/26 opens jaw. Keep iris centering to avoid wall‑eye unless narrative calls for it.

Disgust: AU9 wrinkling at nasal bridge; AU10 raises upper lip and one alar wing; often asymmetric. The nasal under‑plane dominates; teeth may show laterally.

Document min/med/max amplitude frames for each, so animation knows safe ranges before caricature.

8) Phonemes & Speech Coordination

Include a viseme row for A, E, F/V, M, L, OO/W. Note how each interacts with emotions (e.g., smiling speech maintains AU12 while corners still follow phoneme arcs). Mark jaw vs lip contribution percentages to guide rigs (e.g., /A/: 70% jaw, 30% lip; /F/: 10% jaw, 90% upper‑lip press against teeth).

9) Topology & Rig Handoff

Provide a callout layer that shows:

  • Orbit loops around eyes;
  • Nasolabial flow and mouth corner spiral;
  • Chin pad loop for AU17;
  • Alar wing geometry separated from the cheek;
  • Brow loops that allow AU1 vs AU2 independence.

State which deformations are geometry‑driven (primary AUs) vs texture‑driven (wrinkle maps). Define blendshape groupings by region (brow, lids, nose, upper lip, lower lip, corners, chin, jaw). Include corrective shapes for combinations you know will hit (e.g., AU6+12, AU4+23).

10) Lighting & Material Guidance for Sheets

Keep a consistent three‑point or single‑key setup across the grid so differences are deformation‑driven. Mark material IDs for skin, wet line, teeth, gums, tongue, sclera, iris/cornea, lash/brow, hair. Provide roughness/SSS notes per zone: oily T‑zone, matte cheeks; wet line and tear meniscus higher specular. For stylized projects, lock to two values + accents to keep expression contrast clean.

11) Diversity, Age, and Body‑Type in Expression

Faces with different craniofacial baselines deform differently. A low nasal bridge with fuller infraorbital pads will show less sclera on joy than a deep‑set eye; a wide alar base gives stronger disgust silhouettes; high body fat softens nasolabial compression; age increases brow/forehead fold count and decreases amplitude. Provide variant expression tiles for diverse head types in the project bible so rigs can parameterize amplitude per head family.

12) Common Failure Modes & Fixes

  • Sticker eyes: add lid thickness and ensure upper lid overlaps iris in neutral; move highlights with eye direction.
  • Corner paralysis: mouth corners don’t travel on the cylinder; fix with explicit corner path arrows and min/med/max positions.
  • Nose mush: no defined under‑plane; sculpt columella and alar wedge separation.
  • Mirror‑perfect faces: add subtle asymmetry (brow height, eyelid weight) for charm.
  • Wrinkle noise: too many micro lines before macro plane rotations read; strip back to 2–3 designed folds.

13) Workflow: From Neutral to Grid

  1. Lock neutral with planar clarity and topology notes.
  2. Block AUs individually—study how each rotates planes. Save mini‑swatches.
  3. Compose recipes for core emotions (min/med/max). Validate in front and ¾.
  4. Add visemes and test with one line of dialogue in thumbnails.
  5. Light consistency check; add material accents last.
  6. Annotate with AU labels, corner paths, lid compression notes, and rig flags.
  7. Review at three distances and on dark/light backgrounds for universal readability.

14) Drills & Studies

  • Daily (15–30 min): one AU study (e.g., AU12) on three different head types; one mini grid of asymmetry variants (skeptical brow, one‑sided sneer).
  • Weekly: a 3×4 sheet (neutral + 5 emotions + 6 visemes) for a single character, front and ¾; one lighting pass to verify reads.
  • Monthly: a diversity expression pack (three head families) with amplitude charts and a rig handoff page.

15) Checklists

Design: silhouette communicates emotion • mid‑planes rotate clearly • asymmetry adds charm • AU labels present.

Production: orbit and lip loops clean • alar/cheek separation modeled • blendshape groupings listed • amplitude min/med/max defined • visemes included.

Review: reads at thumbnail • consistent lighting • material IDs annotated • works in grayscale first, color second.

16) Closing

FACS‑informed expression sheets align anatomy, performance, and pipeline. By treating expressions as modular AU recipes, staging clear plane rotations, and designing for appeal at multiple distances, you give animators and riggers a trustworthy map—and you ensure the character’s emotional life remains intact from sketch to shipped frame.