The first thing I've ever heard about Mr. Prowse was that he wasn't told about the big 'father' revelation in TESB, because he had big mouth and was leaking information from the set even when he was repeatedly asked not to do so. Then in some highly disturbing radio (?) interview, the guy who was asking questions was also making incredibly insultive remarks about Lucas (one including "L. needs to take his 'co*k' out of P. a**") and David Prowse was laughingly agreeging with him... and still arrogantly stating "Lucas has to hire me to play Vader in Ep.III."
I'm still bemused every time I recall this...