Okay, so reader k jensen posted this question:
I have a question for you. I have animated for many years and teach animation from time to time. A student asked me one day if I could define the diffrence between the princeples of: Follow through, secondary action and overlapping action?
I have always used those terms to mean the same thing which just proves what a meathead I am! Check out "The Illusion of Life" for a detailed description of these terms. Here's a simplified overview:
Follow through and Overlapping action, according to the book, are two names for the same knid of thing. Basically, it covers all the stuff that prevents a character from looking like it snaps from place to place. For example, the clothes and hair "Catch up" after the body has stopped running. In a fleshy character, their fleshy cheeks might take a moment to catch up to the skull in a fast head turn. Or it could even apply to appendages. As you stop running and your feet get planted, your arms and head might continue to move for a bit until they settle.
Also in this category are things called "follow through" in real life. Like when you swing a baseball bat at a ball - after the bat connects you still continue to swing the bat until it comes to a rest.
Secondary action is "extra business that supports the main action...and is always kept subordinate to the main action". Like a flustered person who puts on his glasses as he regains his composure. The Illusion of Life continues: "If it conflicts or becomes more interesting or dominating in any way, it is either the wrong choice or is staged improperly".
Make sense? If not, please see the "illusion of Life" for a better explanation and some examples! Hope that helps, k!