The most important principle is called squash and stretch. When an object
is moved, the movement emphasizes any rigidity in the object. In real life,
only the most rigid shapes (such as chairs, dishes and pans) remain so
during motion. Anything composed of living flesh, no matter how bony,
will show considerable movement in its shape during an action. For
example, when a bent arm with swelling biceps straightens out, only the
long sinews are apparent. A face, whether chewing, smiling, talking, or
just showing a change of expression, is alive with changing shapes in the
cheeks, the lips, and the eyes.