secondary action

To make a primary action of character more life-like, support it by adding secondary actions to the character. These are actions that are usually unnoticable, but without them, the action doesn't look convincing.

For example, when a character is walking, he or she is usually also swinging their arms and doing a facial expression or something verbal (like talking or whistling).

The point of such actions is to empihize the main action that the character is doing. If a secondary action conflicts or becomes more interesting than the main action, then the animation fails.

Facial expressions are generally considered secondary actions, so its best to show changes in expression when a character is stationary or before or after a character moves (not during the movement, otherwise the viewer won't notice the change).