In response to criticism towards Hit-Girl's character, Chloë Grace Moretz stated in an interview, "If I ever uttered one word that I said in Kick-Ass, I would be grounded for years! I'd be stuck in my room until I was 20! I would never in a million years say that. I'm an average, everyday girl." Moretz has said that while filming, she could not bring herself to say the film's title out loud in interviews, instead calling it "the film" in public and "Kick-Butt" at home.
After being rejected by every studio they approached, Matthew Vaughn raised the budget at a dinner party and made the movie independently. Vaughn ultimately sold the movie to Universal for more than he had originally asked them for.
The credits give descriptive names to several of D'Amico's "goons" for correct character attribution. Five of these goons, listed as "Posh", "Scary", "Sporty", "Ginger", and "Baby", were named for each of the Spice Girls.
Big Daddy's line, "Now go to Robin's revenge!" is a reference to a Robin comic where Robin was fighting a blind master martial artist. Even though he was blind, his skill was such that Robin could not lay a finger on him. Robin was able to set up a whistle to blow automatically. This overloaded the blind man's primary sense and allowed Robin to sneak up behind him.
Mark Millar and Matthew Vaughn knew each other socially through Jane Goldman and her husband, but came to work together because of Thor (2011). Millar had sent Vaughn some suggestions for the Marvel adaptation, but Vaughn ultimately left that project. He came to Millar to see if he had any interesting ideas, nearly leading to a sequel to the Bible called "American Jesus". As they were discussing that, Millar mentioned Kick-Ass, and started sending Vaughn the early script for the comic.
Nicolas Cage modeled his speech mannerisms as Big Daddy on original Batman (1966) Adam West. According to Matthew Vaughn, Cage started acting out his lines this way at the first costume fitting. The director was happy for Cage to continue with this performance in the film, citing his irritation with the gravelly voice Christian Bale used in The Dark Knight (2008).
The name of the lead character Dave Lizewski was chosen at a charity auction run by Kick-Ass co-creator/writer Mark Millar prior to the publication of the 1st issue of the book. The auction winner chose his own name to be used.
The original Dutch certification for this movie was 12 years. After several complaints and due to the language and violence in the movie, the Dutch Institute for Classification of Audiovisual Media (NICAM) re-rated the movie for 16 years and older in May 2010.
In the original comic the Mark Strong's character is named "John Genovese". Genovese is one of the most powerful "Five Families" (Bonano, Colombo, Gambino, Genovese and Luchese) of the mob, and to avoid any trouble with them, the name was changed to Frank D'Amico, at the same time doing a nod to The Simpsons (1989), because the mob boss of Springfield, Fat Tony, is called Anthony D'Amico.
A pair of screenprints seen in Amico's apartment depicting a revolver is a 1981-82 piece titled "Gun" by Andy Warhol, depicting how the gun has become a significant part of the American society, following an attempt on his life by Valerie Solanas in 1968. This piece was produced only with the revolver (similiar to what Solanas used in the near fatal attack) shown on the right side.
Production designer Martin Childs was fired by director Matthew Vaughn early in the production process because they were "not on the same wavelength". Russell De Rozario was hired as his replacement five days before shooting started.
The opening guitar notes of track "The Corridor", played before Hit Girl kills the thugs in the corridor, is based on "Kryptonite" by rock band Three Doors Down. The song is based on the fictional radioactive material in the Superman franchise, which is the titular character's primary weakness.
'Dexter Fletcher' plays a gangster called Cody who drives a yellow Range Rover. In director Matthew Vaughn's previous film Layer Cake (2004) he also plays a gangster called Cody who drives a yellow Range Rover. It is unclear if they are the same character.
The trivia items below may give away important plot points.
Red Mist" is a term that has been used to refer to a bomb-disposal person who gets blown up by the bomb they were trying to defuse. This is (basically) what actually happens to Frank D'Amico, the father of the "Red Mist" character, near the end of the movie.