John Wayne was diagnosed with lung cancer on 9/13/64, shortly after filming ended. He suggested that Kirk Douglas should replace him in The Sons of Katie Elder (1965) if he did not survive surgery.
The climactic battle with the Japanese fleet was staged mostly with model ships. Kirk Douglas thought the special effects were poor and complained to director Otto Preminger and the studio about it. He offered to re-stage the scenes at his own expense, using the special effects people who worked with him on Paths of Glory (1957).
John Wayne was suffering from lung cancer, and by the end of filming he was coughing up blood. Despite that, he continued to smoke up to six packs of cigarettes a day throughout filming. Two months later, his entire left lung and several ribs were removed.
The naval battle sequences were done with models that were so large in scale (for the sake of greater detail), that they could be operated from the inside.
In addition to John Wayne, Franchot Tone was also suffering from lung cancer during filming. Unlike Wayne, who would survive his first case of cancer (dying in 1979, of stomach cancer), Tone battled his first case to his own death in late 1968, with only four movie roles between this movie and his death. Wayne had 18 movie roles, and several television appearances (after having one cancerous lung and two ribs removed on 1/17/64) between this movie and his own death.
Jerry Goldsmith: Early in the film, prior to the Pearl Harbor attack, the composer can be seen as the pianist signaling the band to stop playing.