Why Animators Need to "Progress the Action"

Brer Fox - copyright Disney

One of the most common mistakes made by junior animators is not progressing the action in their animation. In other words, making animation that is too static and immobile.

Progressing the Action is a concept developed in the Animator's Survival Kit, in which Williams describes it as the "secret of lipsync". But what does progressing the action really mean? And how does it work in practice?