Today, it’s one of cinema’s best-known reveals: Darth Vader is Luke Skywalker’s father. But 40 years ago, Mark Hamill was one of the very few who knew the big twist in “Star Wars: The Empire Strikes Back”.

Just a few days following the film’s 40th anniversary, Hamill revealed he was sworn to secrecy to protect the big moment as one of only three people who knew Skywalker’s parentage. His co-stars and crew didn’t find out until they watched the movie for the first time.

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“The cast & crew first learned of it when they saw the finished film,” Hamill tweeted on Sunday. “When we shot it, Vader’s line was ‘You don’t know the truth, Obi-Wan killed your father.’ Only Irvin Kershner, George Lucas, and I knew what would be dubbed in later. Agony keeping that secret for over a year!”

Hamill, 68, previously shared that he had to keep the secret during a 2017 appearance on “The Graham Norton Show”. The fake-out in the script had Alec Guinness’ Obi-Wan Kenobi revealed as a Dark Side villain.

“By the second film, that’s when the scrutiny became more intense, and there was a wonderful substitute revelation in the scene,” Hamill told Norton. “The idea of Alec Guinness being the real villain, I thought ‘Wow, what a spectacular twist!'”

Screenwriter Lawrence Kasdan told EW that everything in the film was leading up to that pivotal moment. that would only be revealed when it was released in theatres.

“All this energy and all this storytelling that proceeded it was aimed at this tiny spot,” Kasdan said in an interview last year. “The filmmakers are focused the entire time on this one revelation, which we not only want to pull off in the best way possible, but we want to keep it hidden from the world to the release.”

Watch the iconic scene below.