Mark Hamill Reveals Star Wars Cast Had No Idea About 'I Am Your Father' Scene Until Movie Was Out

Claudia Harmata

Mark Hamill had a big secret to keep!

Over the weekend, the actor — best known for his role as Luke Skywalker in the Star Wars film series — revealed to fans that his costars didn't find out about the plot twist "I am your father" scene between his character and David Prowse's Darth Vader until the movie's official premiere one year after filming was wrapped.

Hamill's revelation came on Sunday after a fan tweeted at him, praising his acting in the scene and asking what it was like to be on set for the filming of it.

"'Luke, I am your father' scene from The Empire Strikes Back. It's one of the best moments from #StarWars & @HamillHimself's acting is so brilliant during this," the fan account wrote. "I would've also liked to have seen the cast/crew's reaction when they first heard about this epic plot twist."

20th Century Fox/ Everett Collection

RELATED: Mark Hamill Mourns the Loss of 'Deeply Gifted and Beloved' Star Wars Actor Andrew Jack

The actor, 68, replied, "The cast & crew first learned of it when they saw the finished film. When we shot it, Vader's line was 'You don't know the truth, Obi-Wan killed your father.' Only [director] Irvin Kershner, [creator] George Lucas & I knew what would be dubbed in later. Agony keeping that secret for over a year!"

?s=20

Never miss a story — sign up for PEOPLE's free daily newsletter to stay up-to-date on the best of what PEOPLE has to offer, from juicy celebrity news to compelling human interest stories

Several fans who hadn't known this tidbit were shocked and surprised. Actress Melissa Fumero, known as Amy Santiago on Brooklyn Nine-Nine, commented, "WHOA! I did not know that. Amazing!"

"This might be my favorite movie trivia fact every, so great!" another fan wrote.

A third replied, "I will make it my life mission to make sure my child does not know that twist until we watch the film. It is the greatest film twist of all time."

RELATED: New Star Wars Movie in Works from Jojo Rabbit Oscar Winner Taika Waititi

Hamill previously revealed his big secret in 2017 while appearing on The Graham Norton Show, telling the host that Kershner had told him the big plot twist.

"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!'"

However he soon learned that Lucas and Kershner had an even bigger one in mind.

"The director, Irvin Kershner, took me aside, he came to my dressing room actually, and he said, 'I'm gonna tell you something. I know it. George Lucas knows it, and when I tell you, you'll know it. But if it leaks, we'll know it was you,'" Hamill recalled. "So I said, 'What? What?' and he handed me the piece of paper that said, 'I am your father.'"

"I was shocked," he continued. "I said, 'Is that true?' and he said, 'Well, you search your feelings, and we're gonna play it like yeah, it is true!' I said, 'Oh my gosh, Dad Vader!'"

Hamill then also recalled the hilarious reaction costar Harrison Ford had when the cast and crew finally saw the twist at the movie's first official screening.

"So at the screening, when that happened, Harrison turned to me and said, 'Hey kid, you didn't f------- tell me that!'" Hamill recalled with a laugh.