The problem with acting in the theater, as opposed to film and television, is the live audience, Emily Mortimer said recently at the modest Amagansett farmhouse she and her husband, Alessandro Nivola, bought five years ago. “Everyone in the audience has paid for a ticket and suspended their disbelief; they’re counting on you and you’ve got only one shot. I’m always afraid I will break the illusion by shouting something like ‘Fuck the Queen.’ ”