Casablanca is a genuinely miraculous film, one of the few Hollywood masterpieces that really was created by committee. The script incorporated the work of six principle writers, who had lots of editorial supervision. One of the film's most memorable lines, "Here's looking at you, kid," was reportedly contributed during filming by the actor who spoke it, Humphrey Bogart, and supervising producer Hal B. Wallis wrote the famous last line, "Louie, I think this is the beginning of a beautiful friendship", which was added as a wild line after principal photography ended.
If the film has a nominal auteur it would have to be Wallis, who organized the collective that made the film and generally had the last word on what became part of the final product. Jack Warner, the head of the studio, made only one creative suggestion -- to cast George Raft in the Bogart role, an idea that would have made the film the instantly forgettable potboiler it might easily have become. Wallis talked Warner out of the idea, but he had some bad ideas of his own, including turning Sam into a female African-American -- but Wallis himself got talked out of these ideas in turn.
The film's director, Michael Curtiz, was known as a great "director of scenes", with a sure sense of pacing, but he spoke English badly and apparently had no sense of story construction. It was Wallis who "constructed" Casablanca.
One of the delights of the film is its multifaceted quality. The play it's based on provides the milieu of the story and some of its dramatic highlights, but has none of the elements that make the film an enduring classic. The screenwriting Epstein brothers provided most of the witty dialogue and writer Howard Koch pushed its political themes to the fore, but I would argue that it was another writer, Casey Robinson, who didn't receive credit, who supplied the glue that made Casablanca cohere.
It was Robinson who wrote the principal love scenes between Bogart and Bergman, developing Bergman's character into the emotional center of the film. He gave Bergman the opportunity to supply the film with its heart. Without Bergman's performance the film would be nothing more than a diverting programmer with an admirable "message".
The sheer acting craft on display in Casablanca is stunning, but most of it is just that -- craft. Bergman brings an emotional commitment to her role that's of a different order. She suggests an inner life that is mysterious, complex, fully rounded. It's through her eyes that Bogart becomes sexy, that Henreid becomes admirable, that the dangers of Casablanca become real.
The film's narrative promises much in the way of romance and intrigue and adventure, but Bergman is all those promises fulfilled. Audiences loved Bogart and accepted him as a romantic leading man because he held his own with Bergman in this film, tried to expose himself to her emotionally on her level and often enough succeeded. Study his expression, his eyes, in the very brief close-up of Bogart taking his last look at Bergman's face on the airfield -- it's devastating, a moment of total exposure. By the same token, we recoil at Henreid's Victor Lazlo because he never opens himself to Ilsa, because he stands on idealism and form even when gazing into her miraculous eyes.
Roger Ebert has pointed out how Bergman could paint an actor's face with her eyes -- we can see her trying to penetrate his being, and in the process she gives him being. It's the alchemy of romantic love incarnated. We instinctively despise any leading man who doesn't treasure her for this, we instinctively admire any leading man who does.
The ending of Casablanca is morally thrilling, glamorizing virtue and sacrifice, but it would be little more than a literary gesture without Bergman's presence, without Bogart's appreciation of her presence. His sacrifice of it breaks our hearts over and over again because we feel it as our own sacrifice. By that point in the film she's become every great love that anybody has ever lost and we hate to see her go -- always have, always will. Ingrid Bergman is the true author of the miracle of Casablanca.