It seems every time Hammer is slated to wear anything other than tailored clothing there's a problem. With the Snow White film, it was merely the absence of a chest-hair rider in his contract. But his role as Batman in George Miller's Justice League dematerialized in pre-production in 2008 when the movie was scrapped. The same could happen with the title role in Gore Verbinski's on-again, off-again Lone Ranger reboot. Luckily, starring opposite Leonardo DiCaprio in next month's J. Edgar, he wears mostly FBI-appropriate suits as he portrays Hoover's G-man love interest.

"Banana is the safe word," announces Hammer's body double, Kyle, and suddenly, the bestilted stuntmen begin the fight sequence in Scene 21, in which the sword-wielding prince is attacked by giants. They run through it over and over, each time faster than the last, with Hammer ducking, spinning, pivoting, swirling, and lunging—fending off men on titanium stilts who do flips and split jumps over his head.

After a couple of dozen runs, the stuntmen give Hammer a round of applause. "Okay," he says, "can we work on some aerial stuff? Some side flips?" When he finally stops his tumbling a few minutes later, Hammer's knees are bloody, but he doesn't seem to notice. During the grueling three-hour-long session, he never says banana. Instead, when it's done, Prince Charming is back in the van, cooing into his phone. "Hi, sweet wife," he says. "Hi, perfect soulmate."

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It's easy to imagine how Armie Hammer could have evolved into a real-life Sorkinized Winklevoss. An overprivileged asshole eating meat; an über-entitled prig in a robe. In reality, he was almost an itinerant screwup. When he was 12, his family moved from the Caymans back to Los Angeles, where he had been born. "I went from a place where everybody was friendly to a city with no trees to climb. No crabs to catch. No machete. I had long hair and an accent. I didn't know who Nirvana was. I didn't know what the Lakers were." Hammer changed private schools three times, getting expelled from one for writing his name in lighter fluid outside the building and setting it ablaze. He dropped out of high school to act, possessing the desire but not the work ethic. "I figured it all out when my agent threatened to drop me," he says. "I went to three auditions that week. I worked my ass off for the first time and got all three parts." Where lesser oil heirs have ended up coining the term firecrotch on TMZ (Brandon Davis) or appearing on Celebrity Rehab (Jason Davis) or doing whatever it is that Balthazar Getty does, Hammer has ascended to a higher plane: Just eight months after a stint on Gossip Girl, he was working with David Fincher and Aaron Sorkin in The Social Network, then with Clint Eastwood and DiCaprio in J. Edgar.

Like his great-grandfather, Hammer is proving deft at making the most of a moment. "As a fluke," he says, opening an IPA back on the roof deck of his rental in Montreal, "my great-grandfather hit one of the largest oil reserves in California." This marked the rise of Oxy, meaning, in industrial terms, that Hammer has more in common with the Vanderbilts, the Carnegies, and, yes, the Gettys than he does with the Winklevi. "If you didn't know his name," Aaron Sorkin says, "you'd never guess Armie came from privilege—and I don't think he'd want you to guess it. He's a humble, hardworking actor, a friend who'd jump in front of a bus for you."

Hammer starts in on beer two and his genealogy. "My great-great-grandfather Julius," he says, "founded the Communist Party in New York." But the real macher was his son Armand, a doctor who traveled to the Soviet Union in 1921 with pharmaceutical supplies after a typhus outbreak. Because of the Hammers' party ties, Lenin requested a meeting. "Armand became tight with Lenin," Hammer says. "We have a lot of written letters: 'Comrade Hammer, How are you? I have missed your face.'"

The scion of Vladimir Lenin's oil-baron pen pal has a tattoo: his family's name on his inner left wrist in Cyrillic letters. I see it as he punch-cuts a Macanudo cigar. "When Armand was unloading the supplies," he says, lighting up, "Lenin asked, 'What do you want in return?'" Armand wanted art. "So Lenin told Armand: 'Sure, go down to the Hermitage. Take whatever.' And so he walked through the museum. 'I'll take two of those. I'll take one of those. I'll take all your Fabergé eggs.'"