Based on a Dream: Allegedly, the idea for the story came from a fever dream. Many rumours tie the origins of the story to Stevenson's cocaine addiction.
Creator Breakdown: The popular story is that Stevenson wrote the novel after a particularly vivid nightmare. Even if this isn't true, then the novel's themes of duality and of a man suffering drastic personality shifts and driven to ruin by what is essentially an escalating drug addiction probably rang true with the author, who was not unfamiliar with such situations.
Life Imitates Art: Jekyll theorizes that every man may be more than just two men, each with their own role. Roughly thirty years later, Sigmund Freud publishes the ideas of the id, ego and superego.
Playing Against Type: Ingrid Bergman was cast as Jekyll's virtuous girlfriend Bea and Lana Turner was supposed to play Ivy, the slutty barmaid (and implied prostitute). Bergman, bored with playing righteous heroines, requested that the roles be reversed, and they were.