[stevep98]

But the "I am your father" wasn't in the shooting script anyway. The script said "Obi-Wan killed your father", and it was changed to "I am your father when James Earl Jones" said the line for the voice of Vader. Maybe it is that way in subsequent published scripts but not the original.

[greedo]

They didn't trust David Prowse, so the lines were altered in the shooting script. JEJ then recorded the actual lines.

[ksherlock]

Right... Lucas told Mark Hamill before that scene was filmed so he would ham up his reaction.

[kevinmchugh]

And also so that the secret wouldn't get out

[matt-attack]

Relevant:

https://m.youtube.com/watch?v=ATcaQYh0wPU