In his private diaries, Richard Burton records that he and Nigel Green got into a fistfight during filming. He implies that the reasons were boredom, alcohol and irritation at being "stuck together" on-set for so long, rather than serious animosity.
Kirk Douglas did most of his own horse riding and, at one point, broke his nose attempting a stunt that called for him to make his horse fall. Instead of leaning back in the saddle when yanking the horse's head around to the side, Douglas leaned forward and took the full force of the horse's heavy head right in the face.
One day during an afternoon break in filming, William Holden "entertained" a young actress in his dressing room. Later that day, while shooting one of the final scenes with Don Taylor in the water tower, he looked down and saw his wife standing on the set with a stricken look on her face. Convinced she had learned about the dressing room incident, he climbed down, certain his marriage was over. He was greatly relieved when he realized she had only come to tell him she had accidentally wrecked their car.
In a classic case of a director being emotionally manipulative, John Huston did not inform Claire Trevor about when she was to perform her song solo until the very day it was shot. Trevor was not a trained singer, and she had not even rehearsed the song yet. She also felt very intimidated that she had to perform the song for the A-list actors seated directly in front of her. The result was a hesitant, nervous, uncomfortable rendition--exactly the feeling Huston was hoping to get..
According to writer Arthur Miller, Clark Gable had already seen a rough cut of the movie by the last day of filming, and said, "This is the best picture I have made, and it's the only time I've been able to act." Tragically this was the last completed film for both Clark Gable and Marilyn Monroe. Gable died of a heart attack shortly after filming ended, and Monroe died of an alleged drug overdose a year and a half later.
Glenn Ford confessed years later that he had a lousy haircut for this film..
DeForest Kelley rather famously told an anecdote about the filming of this movie in his later years on the Star Trek convention circuit, about the time Princess Sofia of Greece was visiting the set and Kelley flubbed a scene by falling over a chair onto his backside and saying "Oh, shit," in front of the princess. Henry Fonda told him the following Monday not to worry about it because he had danced with the princess over the weekend and ascertained that she had no idea what "shit" meant. This did not stop Kelley from getting a standing ovation at the commissary when the cast and crew broke for lunch.
This film was physically hard on Alan Ladd - he injured a knee during the shoot and broke a hand on the last day of filming.
The opening prologue states: "The Thirty Years War began in 1618. It started as a religious war - Catholics against Protestants. But in their relentless pursuit of power, princes of both faiths changed sides as it suited them and in the name of religion butchered Europe."