When you are a famous actor, other actors are hired to practice for you. These people are called stand-ins. Stand-ins are Bizzaro versions of the real actors - sharing the same height, build, and complexion of their on-camera counterparts.  Some stand-ins could successfully pass as the sibling or ugly cousin of a star. 

Professional stand-ins are good at standing.  Sounds funny, but see how you fare while standing for the better part of sixteen hours so that the lighting crew can get rid of a shadow on your left cheek.  I bet your knees would sound like Pop Rocks.