I eventually went up to see him, he showed me a lot of conceptual drawings from Star Wars and then he said to me “Look, I’m doing this film which is like a space science fiction movie and I’d like to offer you one of two parts in the movie”. So I said “What are the two parts?” – “Well, the first one is a character called Chewbacca…” and I said “What the hell is that?” He said it’s like a hairy gorilla who goes through the film on the side of the good ones. And I just said “No thanks, you keep it…”
I asked what the other part was and Lucas said “The other part’s the big villain of the film”. So I said “George, I’d like to play the villain!”
https://www.galaxiki.org/feature/darthvader.html
For reals!!