At the funeral for her husband, Humphrey Bogart, Lauren Bacall put a whistle in his coffin. It was a reference to the famous line in the film she says to him: "You know how to whistle, don't you? You just put your lips together and blow".
Howard Hawks had bet Ernest Hemingway that he (Hawks) could make a good film even out of Hemingway's worst novel. Hawks chose "To Have..." and proceeded to win the bet by deleting most of the story, including the class references that had justified the book title, and shifting to an earlier point in the lives of the lead characters.
The most famous scene in the film is undoubtedly the "you know how to whistle" dialog sequence. It was not written by Ernest Hemingway, Jules Furthman or William Faulkner, but by Howard Hawks. He wrote the scene as a screen test for Bacall, with no real intention that it would necessarily end up in the film. The test was shot with Warner Bros. contract player John Ridgely acting opposite Bacall. The Warners staff, of course, agreed to star Bacall in the film based on the test, and Hawks thought the scene was so strong he asked Faulkner to work it into one of his later drafts of the shooting script.
Humphrey Bogart and Lauren Bacall fell in love during production. Director Howard Hawks afterward said that it was actually Bacall's character Marie that Bogart had fallen for, "so she had to keep playing it the rest of her life." However, it has also been said that Hawks---who was a notorious womanizer and who had a fling with Dolores Moran during the shooting of the film--was jealous and frustrated that Bacall had fallen for Bogart and not for Hawks himself. He even threatened to sell her contract to Monogram, a lowly Poverty Row studio.
Hoagy Carmichael played most of his scenes with a matchstick in his teeth. Seeing this on the set at the start of shooting, Humphrey Bogart gave kudos to Carmichael, telling him that the matchstick was a nice touch and would make him stand out in the film. Carmichael was surprised afterward to see a scene being filmed with Bogart and Walter Brennan, both of them chewing matchsticks throughout the shot. They finally revealed that they were having a bit of fun at Hoagy's expense.
Many aspects of Lauren Bacall's screen persona were based on director Howard Hawks' wife at that time, Mary Gross, including her nickname (Slim), glamorous dresses, long blonde hair, smoky voice and demure, mysterious demeanor.
Howard Hawks gave John Huston the climax (a shootout on a boat) that he was unable to fit into the end of this film. Huston used this in Key Largo (1948) as he had been having difficulties coming up with a satisfactory ending.
When Howard Hawks discovered Lauren Bacall, he gave her the choice of working with either Cary Grant or Humphrey Bogart. She was very tempted to work with Grant, but Hawks ended up casting her with Bogart in this film, and one of Hollywood's greatest romances was started.
Andy Williams states in his biography that his voice (at age 14) is the one actually used for Lauren Bacall singing "How Little We Know". However, according to author Eric Lax, Bacall did her own singing, after researching studio call sheets.
Film debut of Lauren Bacall. NOTE: The DVD sleeve notes state that she was 19 years of age at the time, while her dialogue in the film indicates that she was playing a character (Marie "Slim" Browning) who was 22.