Ian McDiarmid
McDiarmid was cast by George Lucas in Return of the Jedi as Darth Sidious, the main villain. Sixteen years later, he came back to the role as a younger Senator and Chancellor in the Star Wars prequel movies. He said that it helped to be much younger than his character the first time he played the part. The prequels had him play two faces to his character: He created an pleasant and likeable character in Darth Sidious' public face as Palpatine.