James McAvoy screentested for the lead role in early 2006, but was initially rejected, as the studio wanted a more "conventional" leading man. He was later recalled when the producers decided that his character was more "the runt of the litter." The studio ultimately changed its mind and wanted someone geeky. McAvoy got the part.
James McAvoy trained to improve his physique for the film's action scenes, and suffered several injuries during shooting, including a twisted ankle and an injured knee.
While on location in Chicago, The Dark Knight (2008) was the neighboring film production, in which Morgan Freeman worked while shooting this film. On one occasion, Mark Millar, the creator, snuck onto the neighboring set to see the Batpod which was used for filming. He was evicted from the set when security and producer Lauren Shuler Donner (who visited the set as well) caught him in the act.
The graphic novel features superhero costumes worn by many characters, but the film eliminates them, except the leather attire worn by Wesley and The Fox.