"You've gotta meet the dwarves," Armie Hammer says. "I mean, the little people. The LPs. I said dwarves because it's Snow White. Don't ever call them dwarves. Don't ever pat them on the head." We're driving around the F1 racetrack on Montreal's Île Notre-Dame, going 20 mph in a Dodge minivan, on our way to stunt training in a building behind the bleachers. "They'll all just start making puke noises," he says, "one followed by the next. They have the most amazing rapport. They're like a family." Hammer, 25, pauses for effect, as he likes to do when telling stories. "And they're such horny bastards, too," he continues. "Singling out women like this: 'You know who she looks like? She looks like that girl from the LPA convention in 2008. Oh, man, she gave the best blow jobs.'"

This is Prince Charming talking, relishing an out-of-character moment. For the past three months he's been brushing up on his noblesse oblige and playing a role that seems all but genetically determined. "If you had to draw a prince," says Tarsem Singh, the director of the still-untitled Snow White adaptation, "you'd draw this guy. But I'm not talking about a Disney prince. With Armie, you know there are undertones."

So far, Hammer's one-for-one in the nuance department. His two-way rendering of the contemporary American alpha-douche in The Social Network was instantly iconic. Dropping Aaron Sorkin's lines like Ivy League daisy cutters, Hammer portrayed Cameron and Tyler Winklevoss, the identical-twin Olympic rowers who claimed that Facebook was snatched from them by their Harvard classmate Mark Zuckerberg. Hammer plays not one complete asshole but two, nailing all the physical and linguistic tics produced by the twins' hulking sense of privilege and entitlement. When Hammer-as-Winklevoss wears a robe, it's as if to say, This is how an asshole wears a robe. When Hammer-as-Winklevoss rips into a burger, it's as if to say, This is how an asshole eats meat.

Hammer-as-Hammer, on the other hand, bounds out of the minivan looking like a member of the Hollister SWAT Team. He's wearing flip-flops, green-and-yellow board shorts, and a slate-gray T-shirt. "You've gotta meet the stunt guys," he says, brimming with enthusiasm. "They're weapons experts, ex-firefighters, rodeo riders, Cirque du Soleil performers, wushu masters. A couple are famous for parkour—you're not even going to believe what they can do on their stilts."

Hammer tosses off some gag parkour moves, then turns to me: "You're doing the warm-up with us. You can't just come to stunt training and not train." He means to be just one of the guys, having never developed the sense of exceptionalism that typically accompanies massive wealth. If his trainers sweat, he's going to sweat, and he wants me to sweat too, just so nobody thinks I'm a hack. Or a dick. The suggestion is protective. Hammer's great-grandfather and namesake, Armand, grew Occidental Petroleum from a three-man operation to the fourth-largest oil-and-gas company in America. But Hammer's self-confidence is gentler than his blue blood might suggest, less type-A, more Caribbean. When Hammer was 7, his father moved the family from Dallas to the Cayman Islands. "My parents started a school and a radio station down there, but I had no clue about that," he says. "I just knew I got to grab my machete, get on my dirt bike, and ride wherever I wanted. If I was hungry, I'd cut down a coconut." Hammer lived in the Caymans for five years. "I don't think I put on a pair of shoes the entire time."

Hammer is barefoot again as we join the stunt team, running in circles around the repurposed auditorium, the floor of which is covered with a giant padded mat. The running turns into running jumping jacks, and running while kicking our knees up high, and running while nearly kicking ourselves in our own asses with our heels. Some pelvic thrusting follows, and, finally, a series of stretches that are harder for Hammer than for everyone else because he's six feet five. He's sweating profusely but seems otherwise unfazed. Actually, he's still thinking about those stilts, which he desperately wants to strap on. "The deal is," he says, his foot over his head, his knee near his ear, "the day this shoot is over, I'm getting up on those things and nobody can stop me." He understands the anxiety and liability issues surrounding what he now is—here, for the first time, a leading man, opposite Evil Queen Julia Roberts and poisoned-apple victim Lily Collins—but he could do without the kid gloves. "They're always like, 'But you're not wearing shoes!' And I'm like, 'Huh? Just shut up and hit me.'"

The stilts, it turns out, are titanium, three feet high, and loaded with compressed air that makes even a simple jump look like an event at the X Games. Four stuntmen are strapped into them, doing front flips, side flips, and 720-degree spins. "These guys can run as fast as horses," says Hammer, taking off his shirt and revealing a tuft of chest hair he recently had to fight to keep. "Julia had a line about how smooth my chest is," he says, "and it's not. So the producers were like, 'Okay, just shave him.' And I was like, 'WHOA. I spent 25 years earning this chest hair. I'm wearing purple tights and a codpiece, so, please, let me keep my manhood.'"