It was a brisk morning in November and Cruise had dropped by from nearby London, landing his helicopter on set as the crew piped in the “Mission: Impossible” theme over the loudspeakers. Cruise just wanted to watch, he told them, but Levy, setting up a scene involving a lightsaber duel in the water, suggested the star jump on one of the cameras. He’d meant it as a joke. But there was Cruise, wading into the muddy pond, holding the camera like a pro.