Follow Through
When a character stops moving, any appendages attached to its main body will not halt immediately. Ex: a dog with loose skin and flews, tail and fallen ears. As it comes to a sudden stop, its looser parts keep on moving for a very short while, each stopping at a different time.
That's Follow Through: the continuation of movement that happens with the looser parts (appendages) after the main body has stopped.
Overlapping Action
Back to the example of the dog: while it was running, its looser parts also had some freedom to move on their own, they do not stick to the body. When the dog changed direction, these parts continued on their previous line of motion until the were pressed against its body or pulled by it.
Overlapping Action is the superimposition of motions of the carried parts over those that carry them.
Drag
Tired of being used in our examples, the dog decided to quickly go somewhere else. As its main body and four legs started to move, those same looser parts took a little while to follow. They “dragged” behind the rest.
Drag is what happens with appendages of a body when it starts to move: they take a little time to accompany the movement.
Summing up...
All three cases are about the motion of carried parts in relation to the main ones that pull them around:
Action Main Body...
Follow Through Stops.
Overlapping Action Changes direction.
Drag Starts moving.
In “The Illusion of Life”, the authors describe five main cathegories that were identified:
1.Character with appendages (loose clothing, long hair or ears, antennae, etc.) will have these parts still moving for a short while after the torso has stopped.
2.Bodies don't move all their parts at once. They stretch and squash, parts turn, pull and push working against each other. When one stops others may still be moving, like arms swinging after the trunk has halted. To clearly show a character's attitude and feelings her head, shoulders and trunk may stop at the same time, since those are the parts an spectator "reads" to know how they feel. Other parts may take longer to reach their "rest position" for the current pose -- remember, we're talking about settling down at each pose, not final, complete stops.
3.Parts with more tissue like cheeks, bellies, etc. can move noticeably slower than the bones beneath them. This is called "drag" and, well executed, it helps a lot making shapes look solid and organic, life-like.
4.This is fun! In another sense, "follow through" can also refer to what happens to a character after an action. It deserves its own subsection, check it right after this one.
5.The Moving Hold. TODO
Timing...
...is critical to the correct application of Follow Through, Overlapping and Drag.
