An actor may be nervous about performing in front of people, so during rehearsal he switches from foot to foot while delivering lines. If a character delivers a line which suggests that he or she has been thinking, crossing the arms over the chest is not appropriate. Rubbing the chin while pacing is appropriate. Inappropriate movement gives the audience a conflicting message which makes the scene seem unbelievable. As the emotion of a scene changes, the tempo of movement should also change.
Intense moments call for quick movement.
Casual moments call for casual movement. One obstacle of training new actors is getting it into their head they own a certain amount of space on stage. Their space encompasses as far as their limbs can reach. Getting new actors to utilize their space can be difficult. But an actor who uses big movements draws in the entire audience all the way to the back row.