Not all the actors. David Prowse, who played Darth Vader had no idea. Only Hamill knew. There were only four people who knew at that point. Lucas, Kasdan, Hami

Not all the actors. David Prowse, who played Darth Vader had no idea. Only Hamill knew. There were only four people who knew at that point. Lucas, Kasdan, Hamill and Kerschner.