Jake Gyllenhaal: How a cult hero became a Hollywood prince

"More women flirted with me," laughs Jake Gyllenhaal, "when I shaved my head as a marine in Jarhead than when I got big muscles and had long hair in Prince of Persia."

And well he might laugh. After years of acclaim and quiet, unshowy, altruistic performances which helped his co-stars glean attention (Heath Ledger, of course, in Brokeback Mountain and Tobey Maguire in last year's Jim Sheridan-directed drama Brothers), Gyllenhaal is finally on his way to the sort of routine-shattering global success a Jerry Bruckheimer-produced "tentpole" film like Prince of Persia can guarantee, and he is the latest torch-bearer for Hollywood's "smart-throb" club. There are some of it who are older than him (George Clooney, Robert Downey Jr, Matt Damon) but not too many surrounding him, turning 30 as he does in December.

"Do you like my new teeth?" he mugs, showing me his over-white Hollywood gnashers. "Well, it's all done now." He pauses. "I really am proud of Prince of Persia and think it's really great. And I really honestly never say shit like that. I am legitimately excited about the movie and to me that does bring a sense of ease. But I've experienced all different types of things from all different movies I've made and I just don't know what to expect."

The last couple of years have not been especially kind to Gyllenhaal. In 2008, his close friend and Brokeback Mountain co-star Heath Ledger died and, later that year, his godfather Paul Newman, too ("I don't think I've met a more decent human being", he says). In 2009 his parents – Hollywood screenwriter and producer Naomi Foner and director Stephen Gyllenhaal – divorced and that same year he split from his girlfriend of more than two years, Reese Witherspoon.

Still, 2010 should see everything coming up roses for the perpetually almost-really-famous Gyllenhaal. And indeed something seems to have shifted perceptibly in him. His manner in previous interviews has been not unfriendly, but subtly defensive. I ask if he feels different.

"I do feel more comfortable with myself," he smiles, his guard dropped. His hair is short (in the film it's almost shoulder length), he's wearing a beige T-shirt, black jeans and Converse All Stars. Even casually clad, something in his demeanour means business, suggests seriousness, or perhaps he's just ready for some serious levity after little of it, on-screen anyway, these past five years. Films about torture (Rendition), the military (Jarhead, Brothers) and a serial killer (Zodiac) ensured that.

"And I do feel comfortable being a little funny in this film", he admits. "Delivering humour with a British accent is so different. I mean, you have to do very little. Dry humour, particularly. I could say something with an American accent that wouldn't be funny but all of a sudden was witty and wry with a British one. And I loved that Dastan [his character in Prince of Persia] was funny, and cool and fun. There was a clear character here. He got to do fun stuff, but he was a bad-ass."

It helped that Gyllenhaal is famously physical. He's a regular fixture cycling about Santa Monica, and first pumped his body to exaggerated proportions for his part in Sam Mendes's Jarhead in 2005. His body was so pumped while filming Prince of Persia that he apparently had to cut sections out of his leather-look costume just to accommodate his muscles. He trained extensively in Los Angeles, doing parkour, the French discipline that involves impossible-looking leaps up, over and under urban obstacles, and even running sideways on walls. Gyllenhaal does the latter in Prince of Persia, and does it for real, sometimes on wires, and sometimes not.

Reality is the film's most surprising component and not just in terms of its sets (actually built, outside of a computer) and thousands of extras (real, not CGI). Jerry Bruckheimer tells me that he wanted Gyllenhaal for the very reason that "he's a real actor".

Few actors are in the shape Gyllenhaal was before training even started. "And I really wanted to see if I could do this. I wanted to try my hand at it," he says. Gyllenhaal completed substantially more stunts than your average Hollywood actor. "I did take a photo of myself after this big 35ft jump. I was pretty nervous. I put on the harness and looked over the edge, about five times. After I did it I took a photo that I still have. Maybe one day I'll show you. That's when I felt proud."

Gyllenhaal has always been relaxed discussing work, if not much else. He never discussed his relationship with Witherspoon and went to quite lengthy ends to hide it while promoting the film on which they met (2007's Rendition). But their relationship was already the worst-kept-secret in Hollywood. He never spoke of Heath Ledger either until this year, and barely did then. And though Gyllenhaal was apparently the standby Spider Man when Tobey Maguire's back injury threatened production on Spider-Man 2, he speaks in a way which suggests he was not especially enamoured of that project. "A lot of other movies you have a lot of spectacle going on but the characters [in Spider-Man] are sort of, 'stamp this one in, stamp that one out'. There's not a lot of opportunity."

Gyllenhaal has had opportunities galore in life. Indeed, his parents held his bar mitzvah at a homeless shelter because they wanted their privileged son to see how good he had it.

When I ask Gyllenhaal which film-research experience has been most affecting, he says it was for Brothers. "I worked with boys in juvenile hall and the difference between what happened with their life and what happened with my life, well, it does make me believe in some kind of destiny. It makes me believe in what is the order of things. What lessons we've learned. Yeah, my parents were in the movie business and I got involved and said, 'I'm going to do it,' but you've got to have something else besides that."

Gyllenhaal's sister, Maggie, evidently said the same thing as her younger brother. But did he decide early he had a natural aptitude for acting worth truly investigating? He laughs. "It's a balance between knowing you have it and really feeling like you may not."

In fact, the knowledge that he "had it" dates back almost a decade to a tiny film called Donnie Darko, in which he played a confused, slightly unsettling teen with a portentous, over-sized bunny in tow, and another unsettled teen a year later in The Good Girl, opposite Jennifer Aniston.

Donnie Darko attracted a manic following on DVD and retains an effortless sense of cult coolness today. Even back then, almost a decade ago, famed American reviewer Roger Ebert said Gyllenhaal was "able to suggest an intriguing kind of disturbance" while Variety noted "the actor's knack for glib humour suggests a very young Robert Downey Jr."

With Downey Jr now the toast of Hollywood (his Iron Man 2 had America's fifth-biggest opening of any film ever), the parallel is prescient. Downey Jr too was acclaimed early on, as far back as 1987's Less than Zero and, at 27, was nominated for an Oscar for Chaplin. He wisecracks his way through Iron Man 2, to obviously enjoyable effect.

2005's Brokeback Mountain was, in a sense, Gyllenhaal's Chaplin though the film's myriad plaudits were mostly directed to Ledger, whose role was marginally bigger and certainly showier. Britain was one of the few territories to accord Gyllenhaal an award for the piece, when he won the 2006 Bafta for Best Supporting Actor.

Ask him now if he'd still make the film, given the chance to turn back time as Prince Dastan's dagger does in The Prince of Persia, and it's hard to read between the lines of his response.

"Who's to say? I usually make choices based on film-makers and stories, but no matter how good the stories, if you don't have faith in the film-maker it's hard to deliver and give your heart. Ang Lee was a no-brainer but really, whether it's a sort of totally obscure film about two sheep-herders who end up falling in love in Wyoming or a movie like this, it's the people involved who ultimately give you the confidence to do your best work."

And if awards do not come Gyllenhaal's way thanks to his Prince, rewards must. But he is not cashing in his chips just yet. Instead he is currently filming Source Code, Duncan Jones's follow-up to the acclaimed Moon, and recently finished a low-budget romantic comedy with Anne Hathaway for director Ed Zwick. There's also a left-of-mainstream comedy in the can – Nailed, with Jessica Biel, directed by the resolutely unconventional David O Russell, whose next film is Pride and Prejudice and Zombies.

For Gyllenhaal anyway, Prince of Persia is much more conventional. "This kid doesn't know who his family is. He does something because his heart is good, and that heart is seen by someone else. Being seen really deeply is what being part of a family is. When someone sees you and they really see you for who you are, you become family with them in whatever way that is."

Does Gyllenhaal believe in real-life happy endings? "'Happy' is a funny word. How am I supposed to know? But yes, I guess I do."

'Prince of Persia' opens on 21 May