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The actor told Entertainment Tonight that he spent a lot of time in the gym to prepare for the film, in which he frequently appears without a shirt.
He said: "I guess I've gotten buff. There's a lot acrobatics in the movie - a lot of running up walls, and jumping on things and Parkour, so it requires muscularity, but it requires a lot of aerobic ability too."
Gyllenhall, who was cast as the lead in the movie in May, stars alongside Alfred Molina, Sir Ben Kingsley and Gemma Arterton.
He plays a prince who must prevent a villainous nobleman (Kingsley) from possessing the Sands of Time, an artifact that can reverse time and allow its possessor to rule the world.
"I over-prepared myself because I never knew how much they were going to ask me to do, so I just made sure I'd be hopefully able to do anything," he said.
Filming is currently underway for the movie, which is scheduled for release next May.
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