Los Angeles - Channing Tatum lost 6kgs in one day.
The Foxcatcher actor underwent a gruelling regime to portray wrestler Mark Schultz in the movie and admits he and co-star Mark Ruffalo resorted to extreme lengths to get into the shape they needed to be in.
He said: "I dropped 6kgs in a day. You have a ton of water in your skin and stomach and Ruff and I figured out how much weight we could lose in the gym.
"I was not feeling very well after that."
The 34-year-old hunk admires his character from the film because wrestling takes much more discipline than acting.
He explained: "To win Olympic gold, you train from childhood. There's no just jumping in, saying, 'I'm really gifted.' It's a day-in, day-out obsession."
Channing never formally studied acting so views his work on movie sets as his training.
The former stripper told heat magazine: "I had on the job training. Each film has been a school term. It's an expensive education."
Though the Magic Mike actor is enjoying a strong career, he still regrets roles he lost out on.
He said "I've not cried about losing a part but I really, really wanted John Carter. It's one of those that got away."