George MacKay tried to dodge extras in a scene that took hours to set up. So when he unexpectedly collided with them — twice — he kept going.

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.

The Preparation

In the complicated visual conceit of “1917,” multiple takes were seamlessly edited together to make the film appear as if it were made up of a single shot. So Mendes and his cast and crew required extensive preproduction. “The first time we rehearsed this scene, Sam gave me a lovely note,” MacKay recalled. “He said, as much as there’s desperation and it’s intense and what he’s running to is so serious, there’s almost a euphoria to it as well. It’s this kind of utter release.”

The long run-up to filming also allowed MacKay time to get in shape. “I did a lot of sprint training,” he said. “It was about 300 meters, and the first time I ran it, Sam said, ‘Don’t go too fast or you’ll overtake the camera.’ I was absolutely busting it, and I had to do everything I could just to keep up. But by the time we shot it, I was quite fit.”

It was Mendes’s idea that MacKay should begin the scene at a slower pace before breaking into a run. “The first time, I took off,” MacKay said. “I’d forgotten what the scene was about. Sam said, ‘Why don’t you start walking, almost like you can’t believe you’ve done it?’ My character is so set on what he’s doing, he’s almost out of his body.”

The Filming

The pressure was on when the scene was shot near the end of the production schedule. “We knew we had only four or five goes at it,” said MacKay. “All of the explosions were real, and it took five hours to reset the dynamite for each take.”

The filming necessitated complex choreography behind the scenes: Two grips had to transfer the camera midshot from a large crane to a moving vehicle. The crew members were dressed as soldiers since they passed in front of the lens. “It was such a delicate move,” MacKay said. “It was like passing a baton in a relay.”

The first take wasn’t completed because MacKay got tangled up with some of the 500 background actors who were playing the troops. “Suddenly, I thought, ‘Oh, this can go wrong,’” MacKay said. “You want to get one in the bank straight away.”

Take 2 was more successful, despite two unplanned collisions that made it into the final cut. “The rule throughout the shoot was you don’t stop until Sam says so,” MacKay said. “As soon as the collisions happened, they felt inevitable. There’s a grace to the run, but there’s also a reality to the fact that he got knocked about on the way.”

MacKay wasn’t seriously injured by the mishaps. “I got a couple of bumps and bruises, but I was so warmed up, I didn’t feel them at the time,” he said. “There may have been a couple of choice words said, but they came out in the edit.”

The Final Result

Following two more takes on a second day of shooting, MacKay knew he had nailed the scene. “It was the biggest high,” he said. “It was the culmination of months of work. I remember leaping around the field. My heart’s pumping just talking about it.”

The director acknowledged his company’s accomplishment. “Sam made a little speech afterwards, which he didn’t do for any other scenes,” MacKay said. “He just said thank you, and that this was a shot he’d been dreaming of for a long time. And all of a sudden, hundreds of men started chanting his name. It was a huge celebration.”

Still, MacKay wasn’t sure what to expect before he saw the scene for the first time at a screening. “I didn’t watch the takes as we did it, so I had no idea what it looked like,” he said. “I just knew how special it was for everyone during the making of it.”

MacKay felt equally privileged when he viewed the end result. “In a way, that shot is a reflection of the entire film,” he said. “It’s a very simple journey through massive difficulty, but there’s the notion that it has to get done.”

The scene did get done, and MacKay is grateful. “It’s something that will always be with me,” he said. “I can feel it in my heart and in my legs.”