Body
In my opinion, acting is just as much with the body as it is with words. It is a very physically profession, having to display the body in some way, night by night. Professional actors should be extremely comfortable with their body. Studying at even a beginning level form of dance will help serious actors to expand their range of motion. Risk tasking and trust building skills will come in handing for their acting classes and the actors will be able to involve their entire body in a scene, rather than using just their mouths and hands. I am very fond of Vsevolod Meyerhold and his practice of Biomechanics. Engaging the entire body into a scene allows actors to communicate with each other and the audience easier.