The film was shot in various locations in England, primarily in Hertfordshire. The major part of it (the training sequence) was shot at Hendon Aerodrome, about 15 miles north of London, while the besieged chateau was built at MGM's British studios in Borehamwood.

The chateau set was built by a crew of 250 people over a period of four months. The production filmed there for 25 days before the climactic scene of the Dozen blowing it up. It was 240 feet wide and 50 feet high and reportedly built so solidly that it would have taken 70 tons of explosives to bring it down, so a cork and plastic section of the set was constructed to be destroyed on film.

The training segment of the story took two months to film.

The cast learned judo and commando techniques.

Lee Marvin later recalled how director Robert Aldrich instructed his cast to get their contemporary hair styles changed to ones more fitting for the time and setting. Marvin immediately got a crew cut, but many of the others merely got trims to their existing styles. After telling them twice their looks weren't acceptable, Aldrich finally told them they needed either to come in with their hair cut correctly or else call their lawyers.

Marvin had worked with Aldrich before, on the war movie Attack. He found the director "a tremendous man to work with. You knew when you went to work with him you were both going for the same object--a good final print."

Marvin also had high praise for all the men in the film, commenting that everyone was ideally cast "and even when they ad-libbed a scene, invariably it was in character, so all it could do was to help the film."

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."

During production, Art Modell, the owner of the Cleveland Browns, the football team where Jim Brown had distinguished himself as the NFL's all-time leading rusher, told Brown he would have to choose between football and acting. Brown chose the movies and announced his retirement from the sport. Modell later admitted it was a mistake to make Brown choose.

Marvin related a joke Aldrich pulled on Charles Bronson, who was only about 5' 9" and wore low boxing shoes during rehearsal. When it came time to set up the first inspection scene, he placed Bronson between the 6' 6" Clint Walker and the 6' 4" Donald Sutherland. According to Marvin, Aldrich laughed for about ten minutes over Bronson's perturbed reaction.

As shooting ran over schedule, Trini Lopez left the production, whether it was because, as one story has it, Frank Sinatra advised him to quit so he could get back to promoting his recording and performing career or he was let go when his agent demanded more money. Aldrich had his character written out of the story by explaining he had been killed in the parachute jump behind enemy lines.

Commentary on a DVD release of the film claimed the fake inspection scene was originally to be played by Clint Walker, who objected that it was demeaning to his Native American character. It was then given to relative unknown Donald Sutherland.

By Rob Nixon