Some of Red Mist/The Motherfucker's nastier moments from the comic, including murdering children and raping Dave's ex-girlfriend were omitted from the film. Christopher Mintz-Plasse 's reaction to this was literally, "Oh, thank God."
The lines "Try to have fun, otherwise, what's the point?" and "Yeah, there's a dog on your balls!" were both improvised by Jim Carrey.
Mark Millar was actually delighted by Jim Carrey's disapproval of the film's violence. In his own words, Millar described it as "like saying 'this porno has too much nudity.'"
Evan Peters was unable to reprise his role as Todd Haynes because of scheduling conflicts with American Horror Story (2011). Similarly, Lyndsy Fonseca was only available for a brief cameo as Katie Deauxma due to scheduling conflicts with Nikita (2010).