The screenplay is by Enrique Urbizu, Roman Polanski and John Brownjohn. This is one of the strongest assets the film has. Some might say that the film drags on too long. In my opinion it is building the suspense as well as making good use of its time for character development. The one most important part of the screenplay is its' mystery.
Most films give way too much away.
The Ninth Gate allows viewers to come to their own conclusions and leaves a little still unanswered in the end. The mysterious girl never reveals her identity, which is very sexy. Polanski knows how to set up his shots just this way. In one scene, Corso is reading a book and someone walks into the room. The camera is the mystery person's point of view.