Mark Hamill had to keep The Empire Strikes Back twist secret for more than a year

These days, it's one of the most famous moments in all of pop culture: Darth Vader looking Luke Skywalker in the eye and telling him in no uncertain terms, "I am your father." But when The Empire Strikes Back was being made, this climactic moment was still a secret — and a big one at that, as star Mark Hamill recalled on Twitter this weekend just a few days after the film's 40th anniversary.

"The cast [and] 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!"

As Hamill told EW last year, the Obi-Wan fib would've been a pretty decent twist in its own right: "I thought, wow, Alec Guinness, this icon of virtue, is secretly a villain? That’s a great twist.” But since the entire movie is building to the "I am your father" revelation, it really couldn't have gone any other way.

“All this energy and all this storytelling that proceeded it was aimed at this tiny spot,” screenwriter Lawrence Kasdan told EW 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."

The secrecy ultimately paid off, creating an unforgettable pop culture moment that still packs a punch for first-time viewers.

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