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.
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.
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.
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".
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.
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.
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.
Unknown to Lauren Bacall, Howard Hawks had the option to revert to a partial Jules Furthman script which gave some of Bacall's dialogue to another actress should the aspiring actress not work out. It wasn't needed. In fact, shooting continued with Faulkner re-writing scenes to spotlight her character.
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.
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.
Lauren Bacall writes in her autobiography that it was in the third week of shooting that friendly banter between her and Humphrey Bogart turned to something more. At the end of shooting one day, "...he leaned over, put his hand under my chin, and kissed me. It was impulsive - he was a bit shy - no lunging wolf tactics. He took a worn package of matches out of his pocket and asked me to put my phone number on the back. I did." Bogart was 44 years old and in an unhappy third marriage. The relationship with Bacall was obvious on the set, and while it sparked the onscreen chemistry for his movie, Howard Hawks was furious. He warned Bacall away and threatened that the relationship could damage her career - that she could end up at Monogram Pictures. (By some accounts, Hawks was jealous and had designs on Bacall himself). Hawks warned that Bogart would drop Betty after filming was completed, but nothing could be further from the truth. Bogart was divorced and married Bacall in 1945. They made three more films together and remained married until Bogart's death from cancer in January, 1957.
Lauren Bacall was terrified on the set of her first film. Fortunately, Humphrey Bogart was able to put her at ease with humor and acting tips. Bacall had nervous shakes in her first scenes and quickly learned that keeping her chin down and her eyes up kept her head from trembling. It developed into a trademark sultry look.
In the mornings the cast would run through the script (usually fresh pages from Faulkner) while sitting in canvas chairs. Here Howard Hawks and Humphrey Bogart would often juggle or change lines to suit the personalities of the characters. After lunch, the scenes would be filmed.
According to a biography of William Faulkner, he was the sole author of the "second revised final" script, but Hawks changed so much of the story to suit his own style that little of Faulkner's work remained.
Ernest Hemingway's novel was set in Cuba and the Florida Keys in the 1930s and his Harry Morgan was a booze runner. Jules Furthman's early drafts retained this setting. The Office of Inter-American Affairs raised an objection to the filming of the novel because of its depiction of deep corruption and violence in Cuba. Part of the Roosevelt administration's "Good Neighbor Policy" was to encourage positive cooperation among the American nations to discourage the infiltration of Axis influence. The Inter-American Affairs office carefully monitored popular culture, especially motion pictures, and encouraged upbeat depictions of cooperation. Warners and Howard Hawks were not about to cancel the film outright. By most accounts, it was William Faulkner who saved the picture by suggesting a shift to the Vichy-controlled island of Martinique, which was not only out of the influence of the Inter-American office, it also afforded the opportunity to add Gestapo-influenced villainy.