(at around 18 mins) Famke Janssen performed her own stunt when the glass ceiling breaks. It was extremely important for Janssen to remain where she was and very still, yet even with her basically glued to the spot, the shard hit with so much impact that it bounced her head off the table as you can see on close inspection of the scene.
Geoffrey Rush was never meant to look like Vincent Price (star of the original film House on Haunted Hill (1959)). The original screenplay described Stephen Price as a regular looking businessman. Rush didn't care for this, so he suggested that his character look like the film director John Waters. The director agreed to test this look out. After his transformation, he ended up looking so much like Vincent Price the director decided to keep the look.
William Malone got the idea to set the movie in a former insane asylum when he was filming an episode of Tales from the Crypt (1989) in a former asylum and noticed that crew members were running scared out of the basement, not wanting to film there.
(at around 1h 2 mins) The faceless apparition Price encounters in the saturation chamber was originally designed and created by Dick Smith for Ghost Story (1981), but it never appeared in the final cut of the earlier film.
(at around 12 mins) The names William Malone, Gilbert Adler and Valdemar Tymrak appear on Price's guest list before it is deleted. William Malone and Gilbert Adler are respectively the director and producer of the film. Valdemar Tymrak was a serial-killer and evil spirit in Tales from the Crypt: Report from the Grave (1996), which was also directed by Malone.
(at around 12 mins) The original guest list that Price comes up with, before being deleted, has references to other works of horror. Mary Seward is a reference to both Mary Shelley, who wrote Frankenstein, and Dr. Seward from Dracula, who was a doctor specializing in the treatment of the insane. And Valdemar Tymrak is a reference to the Edgar Allan Poe story, The Facts in the Case of M. Valdemar, about a man who finds himself in an undead state after being hypnotized at the moment of death.
The trivia items below may give away important plot points.
When creating the black "evil" that attacks the surviving cast at the end of the film, the animators used images of nude women. They took film of nude women dancing, mirrored the image, and then repeated that image hundreds of times at different sizes to make the spidery shape you see in the finished movie. If you pause the DVD and look closely, you can make out some of the women.
The original shooting script had an alternate ending where Steven Price had died from the Darkness atop the staircase instead of Pritchett and Eddie dies saving Sara from the Darkness instead of Price. When Pritchett and Sara make it outside, they are eventually found by the cleaning crew. There is a news segment of the TV series "Terrifying but True" about the events of the House on Haunted Hill where Sara is taken to an ambulance to have her injured feet tended to and Pritchett is interviewed by the police while paramedics take out various body bags filled with Melissa Marr's severed body parts.
The original shooting script had Christopher Lee as the host of the TV show "Terrifying but True" that Evelyn Price watches the report of the House on Haunted Hill on. In the film, Peter Graves is the host.