From wikipedia - David Fincher was brought into the project very late in its development, after a proposed version written by Vincent Ward (What Dreams May Come

From wikipedia - David Fincher was brought into the project very late in its development, after a proposed version written by Vincent Ward (What Dreams May Come) at the helm fell through. Fincher had little time to prepare, and the experience making the film proved agonizing for him, as he had to endure incessant creative interference from the studio. The film was Fincher's debut in big budget film making, and at the relatively young age of 27 he had to shoot the film without having a definite script. The added weight was also to create a film worthy of the work of the two revered directors that had gone before him, James Cameron and Ridley Scott.