Robin Williams often played practical jokes in the middle of a take during the making of a film. According to producer Christine Vachon, Williams burst out of a doorway completely nude during one take for a chase scene.
After the film premiered at the Sundance Film Festival, Robin Williams overheard viewers say that they forgot it was Williams in the lead role 15 minutes into the film. An observation that made Williams proud.
Near the beginning of the movie, as the Yorkin family is going over their set of developed photos while in their house, they are eating Chinese food and inside of an opened fortune cookie it reads "Someone wants you to be happy". Director Mark Romanek actually searched through 30 fortune cookies before he found one that related to the film and used it in the scene.
Robin Williams is a fan of the Japanese anime series Neon Genesis Evangelion (1995) and specifically asked to have a model from the series incorporated as part of the film. The model itself, contrary to Jake's explanation that it is of a "good guy", is of an unintelligent, creature-like antagonist from the adult-oriented film ending to the series, Neon Genesis Evangelion: The End of Evangelion (1997). The boxed toy is supposedly from Williams' own personal collection.
Trent Reznor of Nine Inch Nails was initially chosen by Mark Romanek to provide the musical score for the film. Romanek, however opted not to use the score Reznor provided. Some of the compositions meant to be used in this film were reworked and released on the album Still by Nine Inch Nails.
Names of other photographers also appear, including: Yoshi Araki (Nobuyoshi Araki is a very well-known Japanese photographer), Bill Owens, 'Ellen von Unwerth' (a popular German photographer), Aaron Siskind, Paul Outerbridge.
In the first showing of the "photo wall", there are 854 photos. 29 high by 31 across minus three rows of 10, 14 and 21. At a later date the wall has 862 photos, same grid minus three rows of 2, 14 and 21.
During the DVD commentary, director Mark Romanek stated that Jack Nicholson was up for the role of Sy before Robin Williams. This is not the first time Williams was up for a role first approached to Nicholson. When Nicholson was stalling to sign on as Joker in Batman (1989) Williams was in talks before casting was finalized.
The trivia item below may give away important plot points.
On the end chase scenes through the hotel, Sy goes through several doors marked 'exit', but after going through the doorways, you see exit on the other side which would indicate running AWAY from the exit. Also, unless routed into a network, door alarms of that style are self contained and only that door's device would be making the sound on battery power.