According to Ben Mankiewicz of Turner Classic Movies, a columnist visiting the set commented on Steve McQueen's irascible temperament by noting that McQueen seemed to be his own worst enemy: "Steve McQueen's character in 'Hell Is for Heroes' seemed to have a little trouble getting along with people. By most accounts, playing that kind of guy wasn't a stretch for McQueen. Time and time again during production, McQueen got in the face of studio executives or Don Siegel, the director, or even cast members. At one point, a columnist was visiting the set, and he mentioned to another observer that Steve McQueen seemed to be his own worst enemy. Co-star Bobby Darin overheard the comment and quickly replied, 'Not while I'm around'."
Bob Newhart said in an interview that due to the film's ballooning budget, Paramount Studios refused to provide more film stock to the set. The production ran out of film stock before filming the scripted finale, but the abrupt ending has helped the film gain a cult audience.
In his autobiography, "I Shouldn't Even Be Doing This", Bob Newhart said that he was offered increasingly large fees for nightclub appearances during production, much higher than the salary he was getting for this film. He really wanted to get back on the road, and would routinely go to director Don Siegel with ideas on how his character could be killed off. Siegel would respond, "You're in it to the end, soldier."
Director Don Siegel did not want to shoot the scene where Bob Newhart's character has a fake telephone conversation with "headquarters" to fool the Germans listening through a microphone planted in the US bunker, believing that it had no place in the story. He was overruled by the studio, however. Newhart at the time was a hugely popular stand-up comic, and a major part of his act was having one-sided phone conversations. The studio ordered that the scene be shot in order to capitalize on Newhart's popularity. Newhart wrote his own lines for this scene.
Temperatures reached 117° F during filming. Some of the day sequences were changed to night so the cast would not collapse from the heat.