A Star Wars fact I have seen a lot of times is that the original actor for Darth Vader, David Prowse did not know his voice was going to be dubbed over by James Earl Jones.
Wookieepedia claims this, with a note that there is no source:
He would speak the lines from within the suit during filming in his Bristol accent. However, as this ultimately clashed with George Lucas's idea about Vader, James Earl Jones was dubbed over as Vader's voice — a fact unknown to Prowse at the time of filming.
Screen Rant also makes the claim:
During A New Hope, Prowse thought that he was also the voice of the imposing villain. Carrie Fisher said in the documentary Empire of Dreams that Prowse's voice was so banal that they would call him "Darth Farmer," which makes it less surprising that George Lucas would choose to cast a separate actor to voice the intimidating Sith Lord.
After discovering that his voice had been replaced with that of James Earl Jones, Prowse was so frustrated that he stopped learning his lines for The Empire Strikes Back and Return of the Jedi. Instead, he would improvise lines, sometimes speaking complete gibberish, forcing his co-stars to respond as if he had said the correct line.
The some articles suggest this was the plan from the start - e.g. Wikipedia and this making-of video, but David Prowse is quoted by The Sun in 2015 suggesting it was originally planned to be re-dubbed by him but the producers didn't want to fly him out to record it:
People thought I was upset that my voice was not used. But I did the voice all the way through the movie.
“It had to be re-recorded as everything was no good.
“But they forgot to get me to do the dubbing before they went back to America and it was too expensive to fly me out there for the half-dozen or so lines I had, so they got James Earl Jones, who did a wonderful job.
Is there a definitive answer to this?