Eli Roth spent a lot of time with Chicago detectives to get the details of the police district (station) correct. In the film in the corner of an open murder case board appears a card that says "We're gonna need a bigger board," a reference to the famous Les Dents de la mer (1975) line "You're gonna need a bigger boat." This was something Roth actually saw on a board in the police captain's office.
In the film, Bruce Willis' character learns to fire a Glock by watching a YouTube show called "Full Metal Tactics," hosted by real life green beret Shawn Vance. Eli Roth named the show as a nod to Willis' co-star Vincent D'Onofrio who starred in Full Metal Jacket (1987).
Eli Roth wanted the film to be a modern, contemporary take on the story and used morning radio DJs Sway in the Morning and Mancow Muller as a Greek chorus to comment on the violence and function as the film's moral compass. Roth wanted the scenes to feel totally authentic, and sent them the situations and let the DJs film and direct themselves and comment how they would naturally as if it was happening. Roth also used viral videos, memes, and the urban gossip site mediatakeout, which director Roth is a fan of. Roth wanted to show the reactions the way they would really happen, making the point that tragic violence becomes a internet meme within hours.
In the film, Paul Kersey and Frank Kersey are brothers and they both are left-handed. They are respectively played by Bruce Willis and Vincent D'Onofrio who are actually left-handed in real life.
Although not credited, Dean Georgaris re-wrote the screenplay from page one with director Eli Roth. There were nine writers on the screenplay by the final shooting draft, and the Writer's Guild ultimately decided to give Joe Carnahan sole credit despite the fact almost none of his dialogue remained in the final version of the film.