He got the acting bug from his sister. "Maggie was always performing, and in so many ways. Then I got to watch Martha Plimpton and River Phoenix with Sidney Lumet, rehearsing Running on Empty," says Gyllenhaal, whose mother wrote the Golden Globe-winning screenplay. "I was 7, and had no idea who these people were. But I knew I was witnessing something magical."

That magic was the discovery of what it takes to tell a story. Gyllenhaal (who named his production company Nine Stories, after the J.D. Salinger collection) says that's his true love. "What success really gives you is the freedom to fail and then try again," he says. That's been a trademark throughout his career, for better and/or worse: Gyllenhaal rarely does takes the same way twice, once prompting his Jarhead director, Sam Mendes, to say, "He can be a bit of a pain in the ass. If he gets a bee in his bonnet, he won't let it go…trying too hard with being absolutely brilliant. He's also the least technical actor I know. He's not an actor who's designed to hit marks."

Gyllenhaal's way of keeping it fresh can befuddle costars on occasion. The far more common result, however, becomes clear in watching his catalog. For over a decade, actors have been doing some of their best work opposite him—Mark Ruffalo, Jennifer Aniston, Tobey Maguire, Heath Ledger. "It's no accident. Jake takes it all very seriously but also has a very light touch," explains Anne Hathaway, his costar in Love and Other Drugs and Brokeback Mountain. "On Brokeback, my final scene was on the phone with Heath, who was in Venice for Casanova. Jake offered to read Heath's lines. On the last take, he changed the line, ever so slightly. That kicked off something in me, and lo and behold, that's the take in the film."

"I grew up on the other side of the camera," Gyllenhaal says. "And yes, I do love making movies as much as being in them. I love actors, watching what they do, and I do love acting off-camera, and how it helps tell the story. But the camera eventually does turn to you, and then it's a very different question. I don't know if I have the answer to it yet.

"I guess you'll see up in Toronto," he says. "It's me acting against myself."

• • •

"No rehearsal, we just shoot," Denis Villeneuve, the 44-year-old Canadian director of An Enemy, yells across the set, a sprawling former glassworks that's become Toronto's main film studio. Villeneuve and a continuity girl study the monitor in front of us. The shot consists of nothing but Gyllenhaal pacing the living room of a modest apartment. By now, I shouldn't be surprised: He's wearing the dull tan chinos from our dinner.

This morning's scene is simple but pivotal: Adam, the professor, enters his apartment, rushes to his phone for messages, finds none, says "Shit," then hears a knock on the front door he's just entered.

Adam's "double," Anthony, the vain bad-boy actor, is on the other side, and their interaction—to be shot after lunch—will be a crux of the film. Adam meeting Anthony. Jake meeting Jake. Gyllenhaal's impulse is to play it big, trying out various interpretations of Adam's unease as he goes to answer the door. For the next take, Villeneuve asks him to try going smaller, and Gyllenhaal dials it back. This doesn't quite do it, either.

Villeneuve is keeping the cameras rolling, even between takes. "When you let it roll," he says, joining me at the monitor, "the actor can sometimes take you to a space you hadn't thought of. Jake is amazing in those spaces."

Gyllenhaal has positioned himself in front of the camera, giving himself an extreme close-up before the third and final take, his bearded face filling the monitor with one expression of suffering after another. Almost instinctively, his hand rises to cover his mouth in horror. As D.H. Lawrence said, "a young man is afraid of his demon and puts his hand over the demon's mouth sometimes."

Ayers has seen it before, in End of Watch. "That's Jake," he says. "He's a genuinely sweet guy, but he's also got this real darkness, this rage he's running from. All the great actors have it, believe me. It's what you do with it."

As the camera rolls, I see exactly what Gyllenhaal does with it: take after take, each one a manifestation of the horror he's unearthed.