James McAvoy screen-tested 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.
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, sneaked 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.
In the office at the end of the film a nameplate is visible on one of the cubicle walls. The nameplate reads "J.G Millar" which is a composite of writer Mark Millar and artist J.G. Jones, on whose comic the film is based.
The first film to use Red Digital Cinema's Red One digital cameras. Several prototypes were used, alongside film cameras, to shoot about ten scenes. However, no Red One footage appears in the final cut of the film.
About 53 minutes into movie, Morgan Freeman's character decodes names from the woven fabric's yarn as strings of 0's and 1's. This is actually ASCII code used in computers and electronic data processing. In ASCII, capital 'A' is represented by numeric value 65, which is written in 8-bit binary number system 01000001, B has ASCII value 66, C has 67 and so on.
Screenwriters Michael Brandt and Derek Haas named several of the movie's characters after people from their college alma mater, Baylor University. Robert Darden, the name of Wesley's first target for assassination, is the writing professor in whose class they first met. (They have used the name Darden for the first victim in several other films, as well: 3:10 to Yuma (2007) and 2 Fast 2 Furious (2003)). Sloan, the character played by Morgan Freeman, is taken from the university's previous president, who was forced to step down under pressure from faculty and alumni.
Vehicles featured include a Dodge Viper SRT-10; 1966 Ford Mustang; Chevrolet Corvette [C4]; the police cars were Ford Crown Victorias and a 2000 Chevrolet Impala 9CI; elevated subway trains; garbage truck; a Chevrolet Step Van; an express train; and a 1971 VAZ 2101 Zhiguli.
The script went through several revisions with the most significant one being the third act that was rewritten by Chris Morgan in mid-2006. The first draft was rejected by Mark Millar because he felt the script was too tame and Americanized. Dean Georgaris also contributed a rewrite and tailored the role of The Fox specially for Angelina Jolie. Finally, original writers Derek Haas and Michael Brandt then put in the final rewrite to even out the voice of Wesley, which was established in the first draft.
At the beginning of the movie, Wesley walks outside his apartment and looks up to see Cross' home. To the left of the home is a telephone pole with a flyer that says "Your Father's" revealing that Cross is Wesley's real father.
In the final library scene, Fox decides to follow the code, and the bullet she shoots has the word 'Goodbye' engraved on it. You can see the word clearly just prior to it entering the first assassin's head.