This is an action that follows a main action OR a secondary supportive action. The first helps to make the action more realistic and believable, it sells the primary action. The second uses tools to reflect their character's thoughts. Both exist to assist communication of narrative.
Overlap
This helps make an action more fluid and lifelike. It involves a lead action and then a following action that "lags" behind. It offsets the action to make it flow better. Overlapping actions can be represented through the body (e.g. head moves forward and body follows) or through apparel (e.g. flowing dress).
Follow Through
This also helps to make an action more lifelike, but where overlap was about motion, follow through is about what happens when the action halts. Loosely connected parts continue to move after the main body has stopped.
To put these into practice, we had to record a video and augment it by animating a loose article over it. I chose to do a beard, and use overlap and follow through for when the head turns to the side quickly. I found this quite hard as working on top of a video meant that any discrepancies in the drawings were more obvious, as well as the fact that there was slight camera movement. I also found it quite challenging to make the drag of the beard realistic and did multiple takes. It's definitely an area I need to practice more. I feel that the beard looks a bit floaty because of how slow it is but how much it moves on each peak. Furthermore I think that the first peak should be higher and longer than the second as the turn that way is faster. I also tried to incorporate a tiny bit of secondary action with the cheek ripples as the hand slapped it, which helped make the slap seem to connect more.