"Baby Oliver" was cast before he was even born. The producers chose a woman who was pregnant with twin boys who would deliver about the time filming began so the baby would genuinely be a newborn baby, as his character is.
One of the rules that Michael Davis observed was that there would be no explosions. Every scene had to be gun-centric.
Paul Giamatti's first experience of handling a firearm. Both he and Clive Owen attended a gun school to familiarize themselves with how to handle various weapons.
Monica Bellucci, who is a polyglot, dubbed herself for the Italian and the French versions of the film.
Hertz's joke ("Why is a gun better than a wife? Because you can put a silencer on a gun.") was lifted by Michael Davis from an NRA website with a jokes section.