Film has much more subversive potential than the novel. In film, characters act and react to one another, but you cannot see inside their heads. You do not know why they are doing what they are doing. You have only your senses to aid you, and the cinematographer can do a great deal to mute the things that really matter by burying them under layers of noise and spectacle and artifice. In a novel, it is difficult to hide the motivations of the protagonist, impossible to do so without calling attention to the fact that you are “pulling a Hemingway,” and even harder to escape the fact that the author must explicitly call out every detail, so that the astute reader must notice them.
Flannery O’Connor and Walker Percy were able to write subversive novels only by writing novels that looked, on the surface, as though they were actually being “subversive” in the particular way that the literary establishment favors. That is, they appeared to be unabashedly calling for nihilism and the the downfall of Western civilization—which, since this was and remains the precise agenda of the literary establishment, hardly seems subversive, in retrospect. They could not hide their Catholic motivations; they could only disguise them as something else. Percy could not have written Dr. Tom More as, say, a hard-boiled detective and gotten away with it.
Casablanca would never work as a novel. You would know too much about Rick’s insides to be surprised in the final scene.