One scene required Lee Marvin to drive an armored truck with Charles Bronson riding shotgun. With cameras poised, Marvin was a no-show. He was eventually tracked down to a pub in Belgravia and was hauled into a car and taken to the studio, where coffee was poured down his throat. When on arrival he fell out of the car, Bronson flipped, "I'm going to fucking kill you, Lee".
Lee Marvin referred to this movie as "crap" and "just a dummy moneymaker", although he enjoyed the film. The movie has nothing to do with war, he stressed, and he was very pleased that he got to do The Big Red One (1980), which mirrored his own wartime experiences. Marvin also said many of the actors in this film were too old to play soldiers.
The scene where one of the dozen pretends to be a General inspecting Robert Ryan's troops was initially written for Samson Posey (Clint Walker). However, Walker was uncomfortable with this scene, so director Robert Aldrich decided to use Donald Sutherland instead. The scene was directly responsible for Sutherland being cast in M*A*S*H (1970), which made him an international star.
Jim Brown later recalled: "I loved my part. I was one of the Dozen, a quiet leader, and my own man, at a time when Hollywood wasn't giving those roles to blacks. I've never had more fun making a movie. The male cast was incredible. I worked with some of the strongest, craziest guys in the business."
Production ran so long that Jim Brown was in danger of missing training camp for the upcoming 1965-1966 football season. As the season approached, the NFL threatened to fine and suspend Brown if he didn't leave filming and report to camp immediately. Instead, Brown held a press conference and announced his retirement from football. At the time of his retirement, Brown was considered one of the best in the game, and is still considered to be one of the NFL's all-time greats.