(Star Wars) Do you think the part where Darth Vader told Luke he was his father?

Question

Was part of the script or did actor David Prowse just throw that in and it was soo good that they just wrote the story around it?

Answer 1

It was part of the script. David Prowse was the actor in the suit. James Earl Jones was the voice.

Answer 2

Actually, there's a detailed story about that.  Prowse and the other actors on set actually didn't know.  Only Mark Hamill was told what James Earl Jones would dub in later.  He was told to act and emote as if that's what Prowse was saying.

As to how early the Vader being Luke's father twist was set, there's some debate about that.  Lucas claims he had the general outline in his head even before Episode 4 was written.  But we also know that he wrote 5 and 6 AFTER 4 was a hit.  The fact that Vader's name sounds alot like 'Dark Father' kind of gives credibility to the idea that Lucas had the idea very early on.