The Facial Action Coding System (FACS) describes visible facial movements using Action Units, or AUs. Each Action Unit represents a specific facial muscle movement or combination of movements.
Patterns of Action Units can be commonly associated with expressions such as joy, anger, fear or disgust, but FACS describes what the face is doing — not what a person is definitely feeling.
A facial expression should not be used on its own to determine a person's thoughts, intentions, honesty or emotional state. Context matters.
Beyond the Face
The body adds context.
Emotional expression extends beyond the face. Gaze, head position, posture, gesture and movement can all contribute to how an emotional expression is perceived.
Face and body cues are best understood together and within the situation in which they occur. A single gesture or posture should not be treated as proof of what someone is feeling.
Self-Conscious Emotions
Shame and guilt are more complex than the basic facial-expression patterns shown above. They do not map neatly onto one universal FACS pattern and often depend more strongly on gaze, head orientation, posture and social context.
A downward gaze, lowered or turned head, and a more closed posture may sometimes accompany self-conscious emotions such as shame or guilt — but these cues are not diagnostic on their own.