Gyllenhaal, 24, jokes that developing his ripped physique – for the Marine movie Jarhead – took little more than sleeping. "The day after I was cast, something happened, and I was huge," the actor says. But seriously: It's really thanks to a grueling regimen that included running and swimming. "Then we went to boot camp," he says.

"I like to think of myself as his Mrs. Robinson," Jennifer Aniston, Gyllenhaal's costar in 2002's The Good Girl, said last month when handing out his breakthrough-acting honor at the Hollywood Film Festival (left). Meanwhile, Susan Sarandon, his costar in 2002's Moonlight Mile, says he has "dreamy eyes," and his love interest in that film, Grey's Anatomy star Ellen Pompeo, calls him "delicious."

Gyllenhaal's look is classic – a blazer paired with jeans; a suit at the Hollywood premiere of Jarhead – with a rumpled edge, namely those devil-may-care tousled locks. However, he'd beg to differ: "I have the most horrible haircuts in my movies," he says.

Though they officially broke up last year, Gyllenhaal and Kirsten Dunst seem more "on" than "off" these days: The pair, who once exchanged promise rings and shared a $1.7 million L.A. home, are often spotted shopping and lunching together. And when he's with her, Gyllenhaal caters to his girl (left, at a Hollywood hotel). "All he wants to do is spend time with her," says a friend.

Whether he's hitting the surf in Malibu or hitting a fly ball at the batting cages, Gyllenhaal is a natural-born athlete. "Exercise is a natural high," he says. He also loves to hit the road. "I'm a big bike rider. I've been inspired by Lance Armstrong."

Gyllenhaal has opened his heart to several furry friends: The actor and Dunst share custody of their German shepherd mix, Atticus, a pup they adopted from the animal rescue group New Leash on Life, and just this month he added another four-legged pal (left) to his brood.

Show business is in his blood: Gyllenhaal's father is director Stephen Gyllenhaal, mom is screenwriter Naomi Foner, and he followed sister Maggie into acting. Plus, his godmother is Jamie Lee Curtis, and Paul Newman gave him driving lessons on a racetrack when he was a teen. Still, he's never taken his family ties for granted. "There's a humility to him," says his Moonlight Mile director, Brad Silberling.