Mexican writer Guillermo Arriaga is best known in the states for his screenplays. He wrote “Amores Perros” and the critically acclaimed “21 Grams.” From his home in Mexico City, Arriaga tells Steve Paulson where the story idea for “21 Grams” came from, and why it was so interesting to have a religious man direct a film written by an atheist that deals with topics like the meaning of life and the afterlife.