Assuming you’ve seen the brilliant making of but for others sake this wasn’t strictly true.

The only person who was told about Vader being Luke’s father was Hamill. Every other script has the line ‘No. I killed your father’.

This was the line uttered by David Prowse under the mask during filming. Only Hamill was told the truth so his reaction would match up to James Earl Jones’ dub. It was a closely guarded secret until the film’s premier!