The film contains several nods to Charles Bronson and the Death Wish series. (in a deleted scene Barney refers to the vigilante as "a Charles Bronson wannabe"; Paul's cat is named Charlie etc). Both the main characters Paul and David have qualities of the type of character Bronson would often play - a tough cop and a vigilante citizen. When Thomas Binder asked how to play and dress his character as a thug, Dennis Vehlen just replied "Do it like somebody Charles Bronson would shoot." The character of Paul is not named after Paul Kersey (Bronson's character in the Death Wish movies), though.
Belinda Balaski was originally going to play Paul's Mom.
Pretty much all the cast and crew worked for free, because of how much they believed in the project. A lot of the cast members also worked as crew on the film, and most crew members (foremost Stephan Franz, Marco Gizynski or Ingeborg Vehlen) took minor acting roles as well.
The characters of Frankie and Dr. Bob, along with a few other small side characters like Igor or Gonzales did not appear in the original script. Though it's never clearly mentioned in the movie, Frankie is Barney's brother in law.
Cindy is the only main character that did not die in any draft of the script.