Here is a simple assignment showing an important aspect of animation.  The anticipation/reaction of a movement helps sell the action to the audience.  The anticipation can help the viewer know that a big movement is coming up, just as the reaction can soften the end of a big movement.  Or it can be a more subtle shifting of weight like when your character picks up one leg and the weight shifts to the other foot before the character takes a big side step (anticipation) only to subltely settle into balance after the big sidestep (reaction).