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.
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.
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.
In the comic, Big Daddy's backstory about being an ex-cop was a lie. He was an accountant who was bored with his life and kidnapped his daughter and made her into a vigilante. Many fans preferred the film version, including Mark Millar, who said that it "would have messed up the structure of the movie". The comic's artist John Romita, Jr. stated that Big Daddy's story in the film "works better stopping short...You love him better in the film".
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.
Aaron Taylor-Johnson and Evan Peters both have the unique honor of playing the same comic book character, Pietro Maximoff / Quicksilver, in two different simultaneously produced franchises. Taylor-Johnson's portrayal for MARVEL studios in Avengers: Age of Ultron (2014) & Peters' portrayal for Fox Studios in X-Men: Days of Future Past (2014) and X-Men: Apocalypse (2016).
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).
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.
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.
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.
'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.
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.
The song composed for Big Daddy's raid in Frank D'Amico's warehouse is based of "In the house, in a heartbeat". The song was originally used in "28 Days Later" (2002) and "28 Weeks Later" (2007) and was composed by Jonh Murphy. Also, suffered slight changes and the name for the Kick-Ass soundtrack is "Big Daddy Kills".
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.
Aaron Taylor-Johnson, Even Peters, Nicolas Cage and Lyndsy Forseca all have played roles based on Marvel characters. Johnson played Pietro Maximoff/Quicksilver as a cameo appearance in Captain America: The Winter Soldier (2014) and then in The Avenger: Age of Ultron (2015). Peters coincidentally played the same character in the X-Men universe in both X-Men: Days of Future Past (2014) and X-Men: Apocalypse (2016). Cage portrayed the part of Johnny Blaze/Ghost Rider in Ghost Rider (2007) and then again in Ghost Rider: Spirit of Vengeance (2011). Lyndsy Forseca went on to portray Angie Martinelli in Agent Carter (2015-).
When the bad guys are about to burn Kick-Ass and Big Daddy during a live broadcast, one of them says "Fire is good! Fire is our friend!". This is a quote from a scene in the Mel Brooks movie Young Frankenstein (1974), which is in turn a parody of a scene in The Bride of Frankenstein (1935).
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.
Dexter Fletcher who plays Cody is crushed in a bright yellow Range Rover. He played a police officer in Layer Cake and stole a bright yellow Range Rover at the end. Layer Cake was directed by Mathew Vaughn.
Evan Peters and Aaron Taylor Johnson star alongside each other; Peters would go on to play Quicksilver in X - Men: Days of Future Past (2014), whilst Taylor Johnson would play Quicksilver in Avengers: Age of Ultron (2015). Despite playing he same characters, it is not the same Quicksilver, Peters is in the X-Men universe and Taylor Johnson is in the MCU.
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.
In Dave's fight with Rasul, he pulls out a taser and hits him in the forehead. This may be a reference to "Batman Begins"(2005) where in a scene where Rachael Dawes is startled by Batman, she pulls out a taser and hits him in the chest.