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.
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.
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.
The inscriptions on the walls and doors in the hospital seem to be all in German, but are misspelled and do, even if spelled correctly, not make any particular sense when translated. The inscription "Gehrinev Hygiene" (seems to mean German "Gehirn") for example is a rough translation of the English "brain sanitation" into German. Maybe the German inscriptions were just supposed to add a more gothic atmosphere to the movie, but quite certainly they are also a reference to an old German Horror movie The Haunted Castle (1921) directed by F.W. Murnau, which also deals with a haunted manor.
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.