Two related principles, one idea
Follow through is the fact that parts of a body keep moving after the body has stopped. Overlapping action is the fact that different parts of a body start and stop at different times. They are usually taught as one lesson because they come from the same physical truth: a character is not a single rigid object, and nothing attached to anything else changes velocity at the same instant.
The practical consequence is that you should almost never have every part of a drawing arrive at its final position on the same frame. When you do, the whole figure reads as a cardboard cut-out being moved around — technically animated, but with no sense of being made of connected, differently-weighted parts.
Follow through: nothing stops at once
A character runs and then halts. Their body stops on frame 10. Their hair does not: it keeps travelling forward for another two or three frames, then falls back and settles. The same is true of a coat, a ponytail, a scarf, loose sleeves, an ear, a tail, a belly, or anything carried in a hand.
Follow through is what gives a stop a sense of mass, because it is evidence that the character was actually moving. A figure that arrives and freezes completely has no history; a figure whose coat swings forward and then drops back tells you it was travelling, how fast, and in what direction.
Overlapping action: parts move on different schedules
Overlapping action is the same idea applied to the whole of a movement rather than just its end. When a character turns to look at something, the eyes go first, then the head, then the shoulders, then the hips. Each part begins after the one that drives it and ends after it too, so at any given frame the body is in a slightly twisted, in-progress state rather than rotating as a block.
This is what animators mean by "breaking the joints" — deliberately refusing to let a limb be straight and aligned at the same moment as the rest of the figure. It is also why a drawing taken out of the middle of good animation often looks strange on its own: the parts genuinely are out of agreement, because that is how real bodies move.
The place this matters most is the breakdown drawing. Your extremes can be clean and aligned; the frames between them are where the offset lives.
Drag, lag, and leading parts
Before animating a movement, decide what leads it. Every motion has a driver, and the rest of the figure is pulled along behind that driver with a delay proportional to how loosely it is attached.
- The driver is whatever is applying the force. For a reach it is the hand; for a turn it is the eyes; for a jump it is the hips; for a punch it is the shoulder, not the fist.
- Rigid parts — the skull, the ribcage, a bone — follow with little or no lag, because they are tightly coupled to what moves them.
- Loose parts — hair, cloth, a tail, soft tissue, anything hanging — follow with noticeable lag and keep going past the stop.
- Floppy ends — the tip of a tail, the end of a scarf, a long feather — lag the most of all, and oscillate for the longest afterwards.
Lag is not a constant. A part lags more when the movement is faster, and a long chain accumulates lag along its length: the base of a tail might trail the hips by one frame while the tip trails by four.
Building a lag hierarchy
A simple and very effective way to plan this is to write down the parts of your character in order of how loosely they are attached, and assign each one a number of frames of delay. Then animate the driver properly and offset everything else by its number.
Even a one-frame offset is visible. If you take nothing else from this principle, delay the head by a single frame behind the body on every stop and start in your shot, and the whole thing will immediately feel less rigid.
The settle: overshoot and oscillation
Loose parts do not glide to a stop. They go past the resting position, come back past it, and converge — a decaying oscillation. Animating this is almost entirely a spacing problem: each swing is shorter and quicker than the last, so the drawings crowd together as the movement dies out.
Two or three overshoots is usually plenty; more than that and the element reads as rubber. The important detail is that the swings get faster as well as smaller, because a shorter travel in the same per-frame time means less distance. If every swing takes the same number of frames, the settle looks mechanical.
Overshoot is not limited to hair and cloth. A character arriving at a pose can overshoot it slightly and come back, and an arm reaching a target can pass it by a few pixels and return. This is one of the most reliable ways to stop an animation from ending on a dead frame.
Working this way with layers
Overlapping action is painful to animate when everything is on one layer, because changing the timing of the hair means redrawing the body too. Splitting a character up means each element can be timed independently — and the loose elements can be animated straight ahead, discovering their own rhythm, while the body stays planned pose to pose.
Build this structure before you start drawing, not after. See working with layers for how to organise a project this way, and keep the split as shallow as you can get away with — four layers you actually use beat ten you have to keep track of.
Common mistakes
- Everything stopping on the same frame. The defining symptom of missing follow through. Delay something — anything — by one frame.
- Uniform lag. If the hair, the hands, and the head all trail by exactly two frames, the figure reads as a single object with a delay rather than a body made of parts.
- Oscillating forever. Five or six overshoots makes a character look like they are made of jelly. Two or three, each half the size of the last.
- Equal-length swings in the settle. Later swings must be quicker as well as smaller, or the decay looks wound up by a clock.
- Lagging the driver. The part applying the force leads. If the hand trails the fingertips, the character is being pulled rather than reaching.
- Adding overlap to the extremes. Keep your keys readable and put the offset in the in-betweens, or your poses will stop reading as poses.
Where to go next
Follow through is the back half of an action; anticipation is the front half, and the two together account for most of what makes a movement feel physical. Arcs is the natural companion to this article, because trailing parts follow curved paths and nothing exposes a trailing element travelling in a straight line more clearly.
For a sequence that uses overlap constantly, the walk cycle is the standard exercise — arms, head, and hips all operate on offset schedules. For pushing the settle further than physics would allow, see exaggeration.