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.
Before ending up in Lionsgate Studios, the producers of the movie tried several other studios who all had the same ultimatum: "We'll take it, if you drop Hit Girl or make her nineteen."
Chloë Grace Moretz trained extensively with a Toronto circus school before filming began so that she could do most of her own stunts.
The name of 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 first issue of the book. The auction winner chose his own name to be used.
Hit Girl's first entrance wasn't working as well as it could, so it was Chloë Grace Moretz's own mother (present on-set throughout filming) who suggested that they revert back to the comic book and have her daughter use the word "cunt."