Credit: Andrew Cooper/SMPSP

Jake Gyllenhaal knows how to keep it real. While filming Prince of Persia: The Sands of Time (out May 28), the actor, 29, stayed in character even after the cameras stopped rolling.

"Jake would chat with crew and visitors in a British accent," a set source tells Us Weekly. "It was funny to hear this American guy talking in an accent when he wasn't shooting. He took it very seriously."

Another thing the single stud took very seriously? Transforming his body with cardio and power circuits. "it's a very physical role," the actor -- who insisted on doing many of his own stunts -- admitted at the WonderCon comic book convention April 3.

The hard work paid off. "He was very proud of the way his body looked," says a source. "He was constantly shirtless."