According to The Marx Brothers biographer Joe Adamson, Harpo Marx was offered $50,000 to utter the single word "Murder!" in this film, presumably to add publicity value to the film by having him speak for the only time on-screen. Harpo declined the offer and never spoke publicly until a concert one year before his death. As he told reporters at the time: "I've spent 25 years creating the illusion that I can't talk. No matter what you write, they won't believe it's me talking. They'll think you made it up."
Hoping to take charge of their film careers, The Marx Brothers financed this movie themselves, under the name of Loma Vista Films. They even did a brief pre-filming tour of scenes from the movie, as they had done with A Night at the Opera (1935) and A Day at the Races (1937), hoping to sharpen the script's comedy.
Near the beginning of the film, the prefect of police gets on his microphone and says, "Round up all likely suspects." If one looks closely at his lips, he really says, "Round up the usual suspects" - a famous line from Casablanca (1942) - but the replacement line was dubbed in later to avoid legal problems with Warner Brothers.
Originally intended as a direct spoof on Casablanca (1942), it was changed to a more original story.
A Hollywood legend claims that Warner Brothers, which produced Casablanca (1942), threatened to sue The Marx Brothers for using the word "Casablanca" in the title. Groucho Marx wrote a letter to Warner Brothers in which he threatened to sue them for using the word "Brothers": "Professionally, we were brothers before they ever were." However, film critic Richard Roeper claims (correctly) that the story is fake. In fact, Warner Brothers never threatened to sue, but merely inquired about the story of the Marx Brothers' film, to make sure there was no copyright infringement. Groucho used the inquiry as an excuse for a publicity stunt. He wrote a series of increasingly zany letters to Warner Brothers (in which he told the studio, "Professionally, we were brothers before you ever were.") The letters were published in "The Saturday Evening Post" to publicize the movie.