Mia Farrow was slated to play Charlotte Randall. Mike Nichols had to fight to let Farrow have the part, due to the film company's hesitancy over her being too controversial at the time (the then current Mia Farrow/Woody Allen trial). She had to take a salary cut but in the long run she had to bow out anyway due to schedule conflicts.
In the scene when Jack Nicholson realizes that his senses are all incredibly acute, when he is in his office building, there is a voice off camera that says "No. I'm not gonna do it." That line is said by the film's director Mike Nichols.
When Michelle Pfeiffer expressed interest in playing the part of Laura, Mike Nichols and the film's screenwriters wanted to make the part stronger and more important to the story, as it was basically a "woman in danger" trope in the original script. Some of the ideas considered were making her into an animal rights activist or a doctor, both of which would have given her a connection to Jack Nicholson's character and expanded on her conflict with her rich father. Ultimately, Pfeiffer accepted the role without it being hugely upgraded because she wanted to work with Nicholson and Nichols.
When Will meets with Alden and his lawyer, in the bookshelves you can see six Robert Jordan books: One copy of 'The Eye of the World', one copy of 'The Great Hunt', two copies of 'The Dragon Reborn', and two copies of 'The Shadow Rising'.
While schmoozing with other guests at Alden's dinner party, Nicholson's character makes reference to "gay senior citizens" as proof that the world has already ended. 16 years later, Christopher Plummer, who plays Alden, would go on to become the oldest actor, at 82, to win an Oscar for Best Supporting Actor, for portraying a "gay senior citizen" in Beginners (2010).
John Williams was originally attached to compose the music for this film. Soon left the project after the film was delayed and needed some time off after winning the Oscar for Schindler's List (1993), as he'd been working on Jurassic Park (1993), Schindler's List and overseeing the Boston Pops Orchestra in Boston.