Gerard Butler stated that the movie was filmed in an actual prison that was still taking in inmates. During certain times of the day, the inmates would be transferring into the prison and looking confused as to why there was filming equipment all over the place. As a safety precaution the film crew locked themselves behind the cages for safety.
While Gerard Butler was originally signed and announced to play the role of Nick Rice, there are divergent stories about how Jamie Foxx took on that role and Butler was re-cast as Cylde Shelton. In one version, Foxx called the producers and asked if Butler would like to play Clyde Shelton instead, as he liked the role of Nick. When the producers approached Butler about playing Clyde, he thought about it for a second and reportedly said "Jamie as Nick... and me as Clyde? That would be awesome!" However, Butler also said in an interview that HE suggested the role switch between himself and Foxx via his role as a producer on the film. Butler also said that he initially regretted that this idea was implemented by the other producers, but added that the entire process worked out well for the project.
Director F. Gary Gray decided to use Del Frisco's Double Eagle Steakhouse as the restaurant that caters Clyde's lunch after dining there several times during filming. The restaurant was also the location of the film's after-party following its screening at the Philadelphia Film Festival.
Portions of this movie were filmed in the former Broad Meadows correctional facility. Since 1995 when a new Delaware County, Pennsylvania correctional facility opened, Broad Meadows has been used for storage, according to the Delaware County Times newspaper. The now-closed Eastern State Penitentiary in Philadephia was used for some prison exteriors.
Bruce McGill is the only actor of the top four male leads who has not been in a comic book adapted film. Jamie Foxx Starred in The amazing Spider-Man 2, Gerard Butler in 300, and Colm Meaney in Dick Tracy.
The security guards at City Hall greet Clyde as "Nomos," which is the company name visible on his janitor coveralls. In Greek, nomos, means law, relating to the idea of dispensing justice. In sociology, the concept of nomos refers to all of the normal rules people take for granted in their day to day activities (Source: Wikipedia). All of these meanings apply to Clyde's choices and actions.
Gerald Butler' interviewed that a scene where a character is murdered by being repeatedly stabbed with a steak bone was so gory that the MPAA gave the film's initial cut an NC-17 rating solely on that basis. When an edited version with much less blood (though still graphically violent) was submitted, the film was immediately granted an R rating.
At the scene where Clyde is torturing Darby, he cuts off his limbs, all in front of a mirror, which is the same method used by Dr. Danco in the second Dexter novel 'Dearly Devoted Dexter' written by Jeff Lindsay.