George Lucas ended up real crazy and angry at Prowse for some reason fathomable only to Lucas.
From what I can gather, Prowse is a likeable enough fellow.
what happen between him and lucas was that he made a joke between the first star wars movie and filming empire strikes back that he was lukes father. this was carried in a fan news letter and published right before the release. The actor claims that he said this before he ever got the script and at the time had said a lot of other goblygook in jest to the person who interviewed him. to this day no one but him and the fan who interviewed him knows if it true. George Lucas was stuck using him because of contract and tried to remove all evidence of him being the actor including digitally removing his face and putting another actors face on the body of a dying Vader in the return of the Jedi
my source is a documentary I viewed this year unfortunately cant remember the title
I have heard that Prowse was miffed when Star Wars came out that his voice was dubbed over by James Earl Jones. However, using his own voice would have been a disaster as he had a high pitched voice that sounded like it was coming from inside a garbage can when in costume. For the sequels, knowing that his voice would be dubbed over anyway, his dialog became a bit random.
The reason?
The reason was because someone framed Prowse as a leaker who gave interviews to the press about Star Wars. Prowse denied it, and it was clear other people were talking to the press. But Lucas thought it was him.