The script originally had no back story about Robert, so Denzel Washington contributed much to the character's background and back story, including McCall having obsessive-compulsive disorder (OCD). In addition to his daily physical and fight training before filming, Washington interviewed several real-life OCD sufferers in order to gain insights on how to play that disorder correctly.
According to the director, during the research for the role as an underage escort, Chloë Grace Moretz interviewed real-life prostitutes. She met an escort who initially told her to gain weight, because in that profession, in order to take care of different men, escorts have to have some flesh. Moretz, who was a minor at the time of filming, changed her diet and gained weight for the role.
Slavi refers to Robert as "dedushka" when they meet at the restaurant. This is Russian for "grandfather."
Teri was originally written for an older actress (someone of 24), but Chloë Grace Moretz fought hard for the role and impressed director Antoine Fuqua so much, he made the role younger. Moretz was 17 years old when the film was released.
The film's test screenings received the highest scores and most positive reactions of any MPAA R-rated Sony film.