In conversation, the 40-year-old director is cheerful and inviting; his movies, to put it mildly, are not. Despite the success of “Amores Perros,” Gonzalez Inarritu had planned to stay in Mexico to make “21 Grams,” a spiritual sequel to his debut. He changed his mind when heavyweights like Penn began tracking him down. If anything, “21 Grams” is even more harrowing–and, for Gonzalez Inarritu and his collaborator, screenwriter Guillermo Arriaga, more personal. In 1996 the director’s infant son, Luciano, died of complications from a lung ailment. Arriaga lost his 2-year-old nephew in a drowning accident. Both men became obsessed by how people cope with loss, and they quickly reached the same conclusion: not very well. “I think our society denies the fact that we die,” Gonzalez Inarritu says. “We close our eyes. We get surgery to stay young. But it only makes us more sad. Like it or not, life is an inevitable chain of losses. I want to tell stories about what enables us to keep living despite that, and that’s hope.”

Both Gonzalez Inarritu and Arriaga came to the film world from vastly different careers: the director was a popular DJ at a Mexico City rock station, the writer was a university professor of communications. It’s a partnership that thrives on friction. “We work well,” Arriaga says, “because we fight a lot.” Let’s hope they don’t make up any time soon.