An idle motion is a motion that simulates different actor motions and moods while the actor is, or is not talking or listening. With the idle motion, the actor can automatically move so that it looks more vivid rather than having a still cardboard with a human image on it. It creates life-like base movements even before you start animating or lip syncing your custom actor.
Because the default idle motion is mild and neutral; you are able to change to another idle motion from the library to show the true mood of the actor.
In CrazyTalk, each time you apply a voice for the actor, the voice causes the actor to move (also known as Auto Motion mechanism). However, if you only need the the idle motion instead of the auto motion, then you can apply voice in Lips-Sync only mode.