Production had to be closed down several times due to Gary Cooper's frequent illnesses. This was Cooper's penultimate movie. He was diagnosed with advanced metastatic prostate cancer the following year.
Charlton Heston was impressed that Gary Cooper still performed his own stunts, including remaining submerged for long periods of time, despite his age and obvious ill health.
Although Charlton Heston was already a major star, he did not mind taking second billing as Gary Cooper was his childhood hero.
Richard Harris so hated the constant delays while making this movie that he refused to return to Hollywood for five years.
The task of bringing this novel to the screen was originally assigned to writer Ernest Lehman and Director Sir Alfred Hitchcock. Lehman eventually went to Hitchcock and told him that he couldn't come up with anything. Hitchcock told him not to worry about it, that they'd do something else. Lehman said, "But what about MGM?" Hitchcock replied "We won't tell them." That "something else" they came up with was North by Northwest (1959).