Secondary Action is a when a animation has a character performing one action and has another action attach to the main action. This often to help fill out the animation making it seem that there is more to the animation then there actually is to it. It is a important principles but isn’t as important as the other principles because it is used mainly to fill out the animation to make it seem more realistic, the only exception for this is when the animation already has enough within it. These secondary actions are often things such as whistling when walking or swinging of a animals tail while it is walking.
A perfect example of this is Wander Over Yonder episode 5:
Watch from 4:52 – 5:02, This is a perfect example due to the fact if you look at the characters when they are running they are also performing another action this is the secondary action, so when Wander and Silvia are running, Wander is bouncing of Silvia’s back and is playing his banjo this is a secondary action. Another secondary action within this clip is when Hater’s men are chasing Wander and Silvia they are also shooting into the air.