The Hangover star is playing Lieutenant Templeton 'Faceman' Peck for the big screen version of the hit 1980s show, and cut out all sugar and salt from his diet while beefing up for the role.
After filming a fight scene with co-star Liam Neeson, director Joe Carnahan called Cooper over to watch the clip - and the star couldn't believe how buff he looked on camera.
He tells Details magazine, "I had to literally transform my body. As the movie progressed, I got in increasingly better shape. There's this one fight scene with Liam Neeson toward the end, where it's, like, the apex of the work. We finished and Joe's like, 'Brother, come here, look at this,' and he played it back, and I swear to God, it looked like my head was digitally superimposed onto someone else's body.
"I was like, 'This cannot be me - that's the way I look?' It was so f**king surreal, because as a kid I only fantasised about looking that way."