“Zero Dark Thirty,” Kathryn Bigelow’s follow-up to the Oscar-winning “The Hurt Locker,” is screening for the first time to a packed theater in L.A. Its star, Jessica Chastain, is waiting to take the stage for a discussion of the film, in which she plays Maya, the CIA agent who spent the better part of a decade leading the hunt for Osama bin Laden. And Chastain, who is playing to sold-out Broadway houses in “The Heiress,” is anxious. She is shaking like a leaf, and her teeth are chattering. Bigelow asks her if she’s cold. “No,” Chastain says. “I’m nervous!”