It's not what you'd imagine, this scene. It's less than a month until the New York City Ballet's spring season opens, and the piece Benjamin Millepied is choreographing is still wobbly. One of the principal dancers hasn't shown up—no one seems sure why. The opening sequence, a mass movement of nearly two dozen dancers, is unfurling chaotically.

But Millepied, principal dancer with the NYCB and one of classical dance's rising choreographers, isn't cracking any whips. Dressed in jeans, a gray T-shirt with a streak of pink at the shoulder blades, and black Converse with the tongues hanging out, he is a friendly, approachable coach, a little like Mr. Schu on Glee. He's also the only person in the dull rehearsal room in the bowels of Lincoln Center who doesn't look sallow against its colorless walls. He has a deep L.A. tan and an off-duty actor's scruff.

"Okay," he says. "Let's try the opening sequence again." The group shuffles into place and begins. False start. Millepied mimes an eek. "I think we left too soon on that one."

The rehearsal goes on in fits and starts. Sometimes Millepied sits slouching on a bench at the front of the room, swinging his legs underneath him, bobbing his head, and counting the beats under his breath. Occasionally, he'll do a loose-limbed demonstration of the finer points of a dance casually but impressively—like Kobe Bryant illustrating how to execute a reverse layup. "Are you okay?" he asks a tiny olive-skinned ballerina. She smiles and waves him off. When she leaves the room a moment later, the dancers' trainer says to Millepied, "She has a cold." He purses his lips and clucks like a sympathetic aunt. "Poor thing!"

If a relaxed, playful choreographer is rare, Millepied, 32, has the potential to become rarer still—that once-in-a-generation crossover star and actress-courting boldface name previously embodied by Mikhail Baryshnikov. The ballet world is an insular one—it can seem almost Mormon-like in its impermeability—and for those on the inside, regardless of their talents or ambitions, breaking out of it can be challenging.

When Millepied entered its folds in 1993, he had images of Baryshnikov's muscular, seductive performance in White Nights (ballet's answer to Footloose) on a loop in his brain. He had spent most of his childhood in a small town in Bordeaux, France, where his mother ran a dance school. The family—including Millepied's two older brothers—lived briefly in Senegal, where the boys' father trained track-and-field athletes for the Olympics. When he returned to France, Millepied began taking classes at his mother's school. He elbowed his way into dance recitals and followed the Paris Opera, France's legendary ballet company, the way a Little Leaguer follows Major League Baseball. "In France, ballet is on TV," he says. "It's on the eight-o'clock news. It's a cool thing to be a dancer." At 13, he lit out for the prestigious Conservatoire National in Lyon and won a spot despite being two years shy of the age requirement. When he graduated, he never considered the notoriously strict Paris Opera school. His telescope was trained on America.

Here, Baryshnikov had desissified ballet's image, having nuzzled with Hollywood royalty at Studio 54 and romanced Jessica Lange. Millepied's future boss and mentor, the Danish-born Peter Martins, had been, after two decades of international stardom, appointed head of George Balanchine's New York City Ballet. And ballet's reigning luminary, Jerome Robbins, was a Broadway legend and Oscar winner who had dated Montgomery Clift. "There was always the sense that something better was going on in the United States," Millepied says.