Despite popular belief, absolutely no wires were used in the film at all. All of the gravity-defying stunts were done through conventional means. For example, the backflip off of the motorcycle was done with a trampoline.
Two different actresses played Preston's wife. The "condemnation archive footage" was filmed first, and featured Alexa Summer. However, when it was time to film the scene where she was arrested, she could not be found. Maria Pia Calzone had to take her place. As a result, there's two separate credits, Summer is credited as "Viviana Preston", while Maria Pia Calzone was credited as "Preston's Wife", even though they are supposed to be the same character.
The "kendo"-type swords used during the sparing scene were designed specially for the film and are turned wood which broke quite easily during filming because Christian Bale and Taye Diggs were really whacking each other with them.
Kurt Wimmer had originally wanted Christian Bale for the lead role as Senior Grammaton Cleric John Preston after seeing him in American Psycho (2000). Bale was not available at the time so he considered Dominic Purcell for the role. Ultimately Bale became available and Purcell was cast as Seamus The Resistance Fighter in the opening of the film instead.
The pistols used by the Grammaton Clerics are Beretta 92Fs (not Beretta M93Rs, as some believe) converted to fully automatic firing. Many other modifications were made, including giving the guns longer barrels and modified slides, top port shell ejection, and adding a compensator to the barrel. Non-firing versions of the pistol were given selector switches. When the gun was fired, the muzzle flash formed the tetragramaton cross symbol - an effect added in post production.
Originally, the drug Prozium was named Librium (hence the term "People of Libria" as often referred to throughout the film). However, Librium turns out to be a registered trade name for the anti-anxiety drug chlordiazepoxide. The name was quickly changed to Prozium, a sly combination of Prozac, and Valium, which both have calming effects.
The film's fight choreographer Jim Vickers and Kurt Wimmer had some slight disagreement on how the martial art Gun Kata should be performed on screen. You can see little friendly jabs to the choreographer in the film. When Kurt Wimmer performs the Gun Kata at the beginning, you see it's very fluid and smooth. The way it's actually used in the film is very rigid with a few flowing moves thrown in. Also, in Angus Macfadyen's speech to the Clerics in training, he says "each fluid position", slightly stressing the word "fluid". Wimmer wanted Gun Kata to be smooth and soft-style. Vickers, trained in hard-style karate among other things, modified the original Gun Kata slightly, and because of budgetary restraints, that's the version we see in the final film. Wimmer has said his own vision of Gun Kata can be seen in most of its glory in his next film, Ultraviolet (2006).
The white sword and ribbon used just before "without incident" sequence was not attached. The intention was that it would be sewn on and that Bale would rip it off at the appropriate moment, but they couldn't get the stitching to work so that it would rip just right. Finally, they decided to leave it unsown, and just have Bale hold it with his hand for the few seconds he was on camera.
The poem that Partridge reads in the church is "He wishes for the Cloths of Heaven" by William Butler Yeats, published in the collection "The Wind Among the Reeds" (1899). "Had I the heavens' embroidered cloths, Enwrought with golden and silver light, The blue and the dim and the dark cloths Of night and light and the half-light, I would spread the cloths under your feet: But I, being poor, have only my dreams; I have spread my dreams under your feet; Tread softly because you tread on my dreams."
Although there appear to be at least two fire trucks (numbered 23 and 14 in different scenes) the production only had one MAN Kat1 A1. The number was changed to give the impression of a fleet of the vehicles.
The trivia item below may give away important plot points.
John Preston kills 118 characters in this movie, which makes him responsible for exactly half of the total 236 deaths shown. As of 2009, he continues to hold the 3rd place record for most deaths in a movie caused by one character.