Kenny Baker was the actor who was in the R2-D2 costume. Unfortunately it was so hard to control from the inside that many times it was just pulled along.

Perter Mayhew was C3PO and was constantly annoyed that people didn’t realize that it was a real actor and not just a robot. Lucas had intended on dubbing his lines with another actor but in the end kept his voice.

Dave Prowse was Darth Vadar and he did not realize that his character would be masked. He was also very unhappy when Lucas chose someone else (James Earl Jones) to say his lines. *** spoiler alert *** Lucas was so secretive about the surprise of Darth Vadar announcing that he was Luke’s father in The Empire Strikes Back that Dave did not find out until the movie premiered in the United States. He was NOT happy. (I also read somewhere else that James Earl Jones was surprised and shocked when he was given his lines for that scene).

(Book – Empire Building: The Remarkable Real Life story of Star Wars – Gary Jenkins)