"Sin Nombre" captures their plight through an accumulation of details that convey the accents, slang, social customs, sights, sounds, smells and music of Mexico and Central America with a precision and authenticity rarely found in U.S.-made movies set in foreign climes. Fukunaga also visited prisons to interview gang members involved in human trafficking, and shelters serving young men and boys who'd lost arms and legs in train accidents.

Kaufman met with former Maras and agencies that work with them in Los Angeles. They shot on gang turf in Mexico, and a few gang members appear as extras.

They don't teach you this stuff at New York University's Graduate Film Program, where Fukunaga began churning out his first short films after earning a bachelor's in history at UC Santa Cruz.

"Cary took the challenge of making a movie in another language, about another culture, incredibly seriously," Kaufman says. "For him that meant that he had to be incredibly authentic, and with people on the set from Honduras checking things that you would never even notice, like the different way that people eat tacos in Honduras and in Mexico."

In casting the film, the crucial decision was to pair Paulina Gaitan, a young but experienced Mexican actress, with Edgar Flores, a much less polished Honduran actor. Kaufman had seen Gaitan in Marco Kreuzpaintner's "Trade" with Kevin Kline, in which she plays the victim of a Mexican sex-kidnapping traffic ring, and felt sure she'd deliver the naturalistic performance that Fukunaga wanted.

Speaking by phone in Spanish, Gaitan says it was challenging to affect a Honduran accent, but "the hardest thing was to get on the trains, because I'm afraid of heights. The first time I did it I was terrified."

However, Gaitan had no qualms about Fukunaga, despite his relative youth. She was particularly impressed at his knowledge of immigration. "When we found that he was a U.S. director and he knew about this, we all thought, 'Wow!' "

Flores turned up at an open casting call in Tegucigalpa and won the part both for his striking looks and ability to convincingly play a tough, if vulnerable, gang banger. "If you look at Edgar's eyes, he's not acting," the director says. "That's just street."

At the beginning, Fukunaga says, "We had to really, like, discipline him, because he would just drink or he'd do whatever. He didn't understand sort of like the opportunity that was given him. But he ended up doing an amazing job."

Fukunaga brought an intense physicality to the set, Kaufman says. "I never saw Cary once sitting in a chair behind a monitor. When Paulina had to get in the river, and the camera guys had to get in the river, Cary got in the river. When someone had to be on top of the train in the rain, Cary was on top of the train, in the rain."

Fukunaga seems hard-wired to take risks and make order out of chaos. No doubt his agent hopes that won't entail death-defying acts for every film. But Fukunaga already is mapping the next journey out of his comfort zone. He's hoping to write and film a modern musical, possibly in tandem with composer and Arcade Fire violinist Owen Pallett.

Metaphorically, if not literally, Kaufman says, "Cary has to ride the train for every movie."

reed.johnson@latimes.com