This film saved the famous love song "As Time Goes By" from being removed from Casablanca (1942). Ingrid Bergman began filming this movie immediately after completing "Casablanca". For this role, her hair was cut short. Meanwhile, for "Casablanca", Warner Brothers wanted to substitute another song for "As Time Goes By" and re-shoot some scenes with Bergman. However, since her hair had been cut, there would be a problem with continuity (even if Bergman wore a wig), so the idea was dropped.
When Ernest Hemingway told Ingrid Bergman she would have to cut off her hair for the role of Maria, she shot back, "To get that part, I'd cut my head off!" She would rehearse tirelessly until all hours of the night, begging to repeat a scene long after the director was satisfied.
Writer Dudley Nichols depoliticized the screenplay, removing all references to Gen. Francisco Franco, loyalists and Falangists. However, he did keep in one prophetic comment about how Germany and Italy were using Spain as target practice.
The book (and movie's title) is taken from John Donne's "Meditation XVII" from 1624: ..."No man is an island, entire of itself... any man's death diminishes me, because I am involved in mankind; and therefore never send to know for whom the bell tolls; it tolls for thee."
At the film's conclusion, Gary Cooper's horse falls and breaks its leg. The only horse the crew could get to do the stunt was brown, but Cooper's horse throughout the film was gray. Rather than re-shoot much of the film, Cooper's brown stunt horse was painted gray.