Casey Robinson, who wrote the screenplay, envisioned Spencer Tracy as the doctor. Tracy and Davis had previously worked together in 20,000 Years in Sing Sing (read about that here). But when Tracy was unavailable, George Brent was cast. Tracy did eventually get the chance to play the part though, when he joined Davis for its radio adaption in 1940.

Dark Victory was Geraldine Fitzgerald's American film debut. However, her role playing Bette Davis' friend was not in the original play. She said, “The character of Ann was to act as a sort of one-person Greek chorus, so that the central doomed figure would not have to cry for herself... This was a wonderful idea and strengthened the drama immeasurably.”

Originally, Reagan’s character was more of a bad influence on Bette’s character. Reagan recalled, “He encouraged Bette to get even drunker. But I felt that this man had real affection for Bette and would have done just the opposite.” Davis agreed with Reagan and got the director to change the plot, which she felt worked better for her character as well.

But it wasn’t easy. Before the film began, Davis' divorce took such a toll that she actually asked to be released from the picture. Producer Hal Willis convinced her to remain, however, and use her emotions to drive her performance. Still, the role itself caused additional grief. “For weeks after we finished filming, I slept badly, and when I woke up, I was too afraid to open my eyes,” Davis said, fearing she would meet a similar fate to that of her character in the film.