Muscles and the Basic Expressions
Art 173a - The Illusion of Life: Facial Animation – Steve DiPaola

There are six basic expression that seem to be universal:
sadness, anger, joy, fear, disgust, surprise.

Paul Ekman compiled a list of every study on facial expression and concluded that these 6 expressions are recognized from a still photograph universally.
Here are there different renderings of the six basic expressions:
6expressions     line drawings     cartoon

Here then is a uneven guided tour through the basic expressions especially there more extreme and interesting variants.

Sadness - -
All adult facial expression have their roots in our very first expression: the scream we made when we first emerged into this world.

The baby's cry - fig1 - Note the tightened eye and square mouth shape.

Adult cry - fig2 - Note the positions of the eye, brows and mouth muscles.

Closed mouth cry -  fig3 - Caught between the pull of opposing muscles.

Anger - -
Anger, the face of rage - fig1  - Note the positions of the eye, brows and mouth muscles.

The brow vrs. the eye, what makes the eyes look intense - fig2  - from neutral eyes, to brows down, then with eyes widened.

The brows down - fig3  - Note the classic V shape of the middle brow area (works even with one eyed creatures!).

Intensity of rage, from medieval archetype to the same expression caught live - fig4 - note: taut mouth/neck muscles and squinting eyes.

Joy - -
The laugh - fig1 - No matter the age, note the similarities: mild closed eyes, nose pulled up, mouth follows teeth, zygomatic pulls up.

More laughing - fig2 - note the same as above muscle-wise, but from the front.

The smile - fig3 - whether iconic, mask-like, or realistic it is universal.

The importance of the squinted eyes in a smile - fig4 - compare the loss of warmth in the non-narrowed eyes rendering.

Fear - -
The face of terror - fig1 - Note the muscles and how they all move out and the wide open eyes.

The difference between surprise, fear and sadness - fig2 - surprise: wide eyes/brows up, fear: wide eyes/brows down, sadness: non wide eyes.

Terror from artist Kathe Kollwitz as death grabs a woman - fig3 - lifted brow / wide eyes / relaxed upper lip / stretched lower lip.

Disgust - -
The face of repulsion - fig1 - brows lowered, squinted eyes, raise checks/nose/upper lip, mentalis pushes up lower lip and wrinkles chin.

Self portrait of a mildly disgusted artist - fig2 - note same features as above but less squinting.

Surprise - -
Surprise with a tinge of joy - fig1

How lighting made Abe Lincoln surprised - fig2 - "from below" inaugural lighting made brow shadows "raise brows" and "widen eyes".

The face "code" of expression - fig3 - We read into the "piece of wood" mask - a surprised expression.
We need the code of both wide eyes and an O mouth. Note how without both the expression code changes:
1: surprised, 2: lost w/ no circles on eyes, 3: wide mouth: still surprised, 4: wide eyes: now astonished, 5: mouth square: makes fear .