When Hollywood makes a movie, there's a lot of work that has to take place before the cameras roll. Lighting has to be set up, the action in the scene has to be mapped out and cameras set up, so the shots will be perfect. People called "stand-ins" are used on set in place of stars during this prep work. The expensive movie stars show up for the action! A stand-in is simply a substitute for the actor who will do the real job.