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 met and 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."
Chloë Grace Moretz fought for the role of Teri when she found out it was written for someone of 24. She presented herself so well, Antoine Fuqua immediately made the change.
Teri was originally written for an older actress, but Chloë Grace Moretz impressed director Antoine Fuqua so much, he made the role younger. She was 17 years old when the film was released.