When I asked Brian Cox, who plays Logan, the patriarch, to describe Strong’s process, he struck a note of fatherly concern. “The result that Jeremy gets is always pretty tremendous,” he said. “I just worry about what he does to himself. I worry about the crises he puts himself through in order to prepare.” Cox, a classically trained British stage actor, has a “turn it on, turn it off” approach to acting, and his relationship with Strong recalls a famous story about Laurence Olivier working with Dustin Hoffman on the 1976 film “Marathon Man.” On learning that Hoffman had stayed up partying for three nights before a scene in which he had to appear sleep-deprived, Olivier said, “My dear boy, why don’t you try acting?” Cox told me, “Actors are funny creatures. I’ve worked with intense actors before. It’s a particularly American disease, I think, this inability to separate yourself off while you’re doing the job.”