When the film was previewed in January 1974, John Wayne was being treated for pneumonia in London. He became seriously ill after appearing on the interview show PARKINSON, which had been recorded a few days earlier. The illness was considered life-threatening as Wayne was 66 and only had one lung, and he had COPD in his remaining lung. He began coughing so hard that he damaged a valve in his heart, although this would not be diagnosed until early in 1978.
John Wayne formally separated from his third wife, Pilar Wayne, during filming, although they had not lived together since 1967, and never divorced before Wayne's death in 1979.
John Wayne was quoted as saying, "I felt like a change, but I had to stick to action movies, of course, and the tough cop thriller is where all the action is, these days." However, he wasn't happy about making the film, thinking the story hackneyed and the script stilted.