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.
During the course of the film the violence becomes progressively more graphic. In early scenes it's almost "PG" with no blood visible despite people being riddled with bullets. As the movie progresses blood starts spilling and by the end single bullets will spill large volumes of blood. This has the effect of increasing the emotional effect of each act of violence from relatively low at the movie's outset to high by the movie's conclusion. It would seem intended that the viewer would grow in emotional sensitivity to the violence just as the main character does.
(at around 54 mins) The "kendo"-type swords used during the sparring 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.
The puppy used in the movie was a Bernese Mountain dog. The noises it makes (barks, whines, yelps etc) weren't made by the dog, but by an actor who was specialised in mimicking animals.
Kurt Wimmer: 1. (at around 1 min) Silhouetted figure in the Grammaton Cleric introduction with narration, 2. (at around 57 mins) Rebel who is pushed up against the column in the warehouse raid and executed.