Alternate Cast
I recently listened to the director's commentary on the disk, and apparently a number of actors were last minute replacements.
I knew about Stacy Keach being a Nicol Williamson replacement and Scott Wilson taking the place of Michael Moriarty, but apparently Jason Robards was supposed to play Ed Flanders' character (Dr. Fell/the real Hudson Kane), and Jason Miller wasn't slated to be in it at all (he was brought in to play Reno when Wilson replaced Moriarty as Cutshaw).
I wonder what sort of movie we would have had with the original cast. I was very pleased with the end result of course, but seeing Robards and Williamson (one of the best US and British actors of the time, respectively) in the movie would have been especially interesting.