Jake Gyllenhaal plays a 6th century prince who holds the fate of the world in his hands in the sweeping fantasy-adventure 'Prince of Persia: The Sands of Time,' and ET's Thea Andrews got up close and personal with the newly minted action hero on the London set! So, how did Jake get that buff bod?

"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," a playful Jake tells Thea.

The former 'Brokeback Mountain' star trained for months before the arduous Moroccan desert shoot, and has been training daily to maintain his action-hero physique, gaining five pounds of muscle.

"I guess I've gotten buff," he shrugs. "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."

When filming finally wraps, Jake promises with a smile, "It's going to turn into fat and I'm going to be happy."

'Pirates of the Caribbean' blockbuster producer Jerry Bruckheimer is bringing 'Prince of Persia' to life, based on the hugely popular video game and scheduled to open in theaters on May 28, 2010. The story finds Jake as Dastan, a young Persian prince who must join forces with the beautiful and feisty princess Tamina (played by 'Quantum of Solace' Bond girl Gemma Arterton) to prevent a villainous nobleman (Ben Kingsley) from possessing the Sands of Time -- a gift from the gods that can reverse time and allow its possessor to rule the world.

"I'm running away from family and I'm running away from bad guys and I'm trying to prove my innocence," says Jake, trying to keep the play-by-play plot details a secret. "There's so many extraordinary elements throughout the movie."

Watch ET for more with Jake and 'Prince of Persia: The Sands of Time'!