TheBoost said:

I just meant in regards to the royalties. With no face, no voice, and a considerable ammount of doubling for the fight scenes, Prowse was kinda a glorified stand-in. Even Peter Mayhew had eyes showing. Vader's most dramatic moments could have been done with a helmet on a stand.

In fairness, he did also have to learn all the lines well enough for other actors to interact with him in a natural way. I remember from the LOTR extras that people thought Andy Serkis was just a stand in, until they realized how much he brought to the role. You might argue that Serkis did more than Prowse, and I would agree, but if Prowse had not learnt his part well, we might have had a similar problem to that of the prequels, in which actors had to deliver their lines to empty air in front of a blue screen.

edit - looks like SilverWook beat me to it. That will teach me to type something off and on, whilst doing several other activities.