In the climactic moment in the World War I drama “1917,” a British lance-corporal dodges explosions and charging soldiers as he runs across a battlefield in an attempt to deliver an order that could save hundreds of lives. When George MacKay, the actor who plays that determined soldier, first read the scene, “I was completely taken by it,” he said. “I could see it very clearly in my mind.”

Yet he wasn’t prepared for the scene’s impact when he finally saw this best-picture contender on the big screen after months of rehearsals and two stressful days of shooting on Salisbury Plain in southern England. “To be honest, I cried,” MacKay said. “I don’t usually get that emotional watching something I’ve been involved in because I’m more objective, but I found it so moving.”

I asked the British actor to break down how he, the director, Sam Mendes, and other crew members put this sequence together. These are edited excerpts from our recent telephone conversation.