Hathaway revealed she devoted herself to training to "physically transform" into Catwoman.
The 29-year-old told Entertainment Weekly that director Christopher Nolan asked her to be fit enough to perform all her own stunts.
Hathaway revealed: "I had to physically transform. Chris sat me down at the beginning and said, 'Joseph Gordon-Levitt did all of his own fighting in Inception. That one zero-gravity fight? He trained for two months.'"
She joked: "I basically left his office and went to the gym and just came out about five minutes ago."
The One Day actress is now reported to be on a strict diet to play dying factory worker Fantine in Tom Hooper's film adaptation of Les Miserables.
Meanwhile, her Dark Knight Rises co-star Tom Hardy defended his character's muffled voice, which some fans criticised after seeing the trailer. The British actor plays villain Bane, who wears a mask, said: "It's a risk, because we could be laughed at - or it could be very fresh and exciting.
"The audience mustn't be too concerned about the mumbly voice. As the film progresses, I think you'll be able to tune to its setting."
We'll be able see for ourselves when the much-anticipated blockbuster makes its way into cinemas on 20 July. Here's the first trailer showing Hathaway and Hardy in action: