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.
Charlton Heston said in his published book, The Actor's Life: ..."it was Cooper's film. After having borne witness all my movie-going life to the enormous presence he brought to the screen, I should've been able to figure that out. I was lucky to be in it. The experience of working with him and the friendship it created is one of the most valuable I've had in film. He was a lovely gentleman (that sadly outmoded word) and a total professional. There aren't many like him."
Richard Harris so hated the constant delays while making this movie that he refused to return to Hollywood for five years.