The twist at the end of The Empire Strikes Back was shocking to Star Wars fans, but it also threw the cast and crew for a loop.
Even James Earl Jones, who provided the iconic monotone voice of the character in cinematic appearances in the last 40 years, said he was convinced that line was a lie when he first read it in the script.
"When I first saw the dialogue that said, 'Luke, I am your father,' I said to myself, 'He's lying. I wonder how they are going to play that lie out?'" Jones said in a documentary called Star Wars: Empire of Dreams.
Not everyone on the set of the film were in the know, and George Lucas only trusted director Irvin Kershner during the production.
"They kept that line secret: 'I am your father,'" Hamill said. "What was in the script was already a fantastic twist. Vader says [in Darth Vader voice], 'You don't know the truth. Obi-Wan killed your father.' And I played it just as you see it. 'No!' and all that. But [prior to shooting] they pulled me aside and [Irvin] Kershner, the director, said, 'Look, I'm going to tell you something. George knows, I know, and when I tell you, you'll be the third person that knows. So if it leaks, we'll know it's you.'"
Jones also talked about returning to the role for that second film, at first admitting he was excited to flesh the character out.
"I remember on [Empire], when George had a chance to counsel me, he said, 'We don't know what we did right [on A New Hope], so let's just try what we did,'" Jones said in an interview with the American Film Institute. "Naturally, I wanted to make Darth Vader more interesting, more subtle, more psychologically oriented. And he said, 'No. no. What we're finding out is you've got to keep his voice on a very narrow ban of inflection because he ain't human, really.' So, that was the answer."
Jones latest vocal appearance came in last year's Rogue One: A Star Wars Story, where Vader stole the show in the movie's final scene.
The newest Star Wars film, The Last Jedi, opens in theaters on December 15.