Doris Day wrote that she was unhappy making this film, which brought back stressful memories of her early career as a band singer, and also because Kirk Douglas and Lauren Bacall (having dated at one time in real life) seemed to intentionally shut her out, making her feel unwelcome.
At least three times you can see trumpeters using mutes different from those on the soundtrack. For example, in the Christmas Eve scene Rick is shown using a Harmon mute and Harry James' dub of the tune is with a cup mute.
Even while married to pinup-girl Betty Grable, Harry James was notorious for his womanizing. Peter Levinson, in his biography of James ("Trumpet Blues") notes that during the filming of this movie, James contacted Doris Day's manager to ask what he thought were James' chances of "nailing" Doris.
The film contains a reference to homosexuality, although the Hays Office--the industry's censor--required any mention of it be subtle. the character of Amy (Lauren Bacall) is lesbian, which is why Rick (Kirk Douglas) walks out on her telling her that she is "a very sick girl". Many decades later, Bacall told a Turner Classic Movies interviewer that the reference was so subtle (and she was then so young and naive) that she didn't understand until years later that the character she played was supposed to be lesbian.
This movie's trio of stars proved to be remarkably long-lived. All were around to celebrate the 64th anniversary of the movie's release. Since then, Lauren Bacall has died but as of August 2014 Kirk Douglas and Doris Day are still alive.