For his upcoming crime thriller Shot Caller, Nikolaj Coster-Waldau put on a serious amount of muscle to play a prison inmate who gets wrapped up in a criminal enterprise behind bars -- but for one of the posters designed for the film, the handsome star bears a striking resemblance to another leading man: Dwayne Johnson.

In fact, for the Shot Caller one-sheet, it appears as though Coster-Waldau's head has been photoshopped onto Johnson's body from the poster for his crime thriller Snitch -- both of which were directed by Ric Roman Waugh.

The Game of Thrones star sat down with ET's Leanne Aguilera while promoting his upcoming film on Tuesday, and he addressed the hilarious photoshopping controversy that has gone viral over the last few days.

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"Apparently it was a Canadian distributor who thought this was a great idea," Coster-Waldau, 47, said when presented with the side-by-side comparison. "Now I can finally tell you the truth: I have been Dwayne 'The Rock' Johnson's body double for the last 10 years."

"I work out five hours every day and, you know, that's how you get to this point," the actor joked, adding that Johnson made him promise not to tell. "But now that the cat is out of the bag, I feel like [it's OK]. It’s a little embarrassing, because Dwayne is a buddy of mine, but I don't know how it got out."

In Shot Caller, Coster-Waldau plays Jacob Harlon, a family man who gets sent to prison after a fatal car crash. He survives behind bars by joining a gang, where he becomes embroiled ever deeper in the prison's criminal underworld.

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During the course of the film, the character spends his days working out, and Coster-Waldau revealed he ended up putting on 20 pounds of muscle for the role.

"These guys, they work out a lot," the star said of prison inmates. "[My character] had, not only an emotional transformation over 10 years,but also a physical transformation, and we had to get that right."

Some Hollywood stars go on restrictive diets to transform their bodies for roles, but Coster-Waldau says his metamorphosis was achieved through "a lot of eating."

WATCH: 'Game of Thrones' Star Nikolaj Coster-Waldau Transforms For New Movie 'Shot Caller'

"It was fun," he said. "It was a challenge [but] it was good fun."

"It was only boring food though," he added. "It was just meat and meat. And meat."

See the actor's impressive physical transformation when Shot Caller hits theaters this Friday. Check out the video below for more.

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