While Big Daddy questioning Hit-Girl on John Woo was in the comics, it helps that he starred in one John Woo directed film, Face/Off.
The Danza: Christopher Mintz-Plasse plays Chris D'Amico.
Dawson Casting: Christopher Mintz-Plasse (Red Mist) and Lyndsy Fonseca (Katie), as well as several other actors, are in their early 20's yet play teenage characters. Somewhat averted with Kick-Ass himself, Aaron Johnson was 19 when the film was released.
Development Hell: The sequel. It was initially announced to be made for a 2012/2013 release but is on hold due to Aaron Johnson taking time off acting to be with his daughter and Matthew Vaughn directing other projects. It's now set to a 2013 release and to be directed by Jeff Wadlow (with Vaughn's blessing/supervision).
It seems that 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 19." Thankfully, they didn't.
The entire reason Red Mist survives the end of Kick-Ass 2 is because when Mark Millar told Jeff Wadlow (Director & writer of the second movie) of the character's death, he argued that they couldn't kill the character off as he's one of the series' staple characters & they couldn't really do Kick-Ass 3 without the character.
Fake American: Aaron Johnson (Dave) and Mark Strong (Frank D'Amico) are both English. Also, Jason Flemyng and Dexter Fletcher, who play two of D'Amico's henchmen.