David Prowse played Darth Vader's body. He was kind of like Agnew to James Earl Jones' Richard Nixon. The reason he didn't actually do the voice of Darth Vader is because he kind of sounded like a Westcounty milk-man
"You are a traitor and a spy. Now would you like full fat or semi-skimmed?"
This article -- which was published in Sunnyvale, California in 1978 according to retroist.com -- contained an interview with Prowse in the build up to the sequel to A New Hope. Prowse talked about innocuous things like his costume, his bodybuilding and the twist ending to his movie.
"Oh, and it won't be out for thirty years but Bruce Willis was dead all along."