John Gallagher, Jr. auditioned for a role in a previous film directed by James Gunn, but wasn't right for the part. However, Gunn thought he was the best actor he'd seen in an audition, and vowed to work with Gallagher again.
James Gunn was originally set to direct this film from his screenplay. However, when it was time to begin, Gunn withdrew, deciding he didn't want to spend several months working on such a violent film. He was also going through a divorce at the time. and withdrew from the project. He later said that he was surprised the movie was moving forward with Greg McLean as the new director. He was happy that he hadn't been asked again to direct, and he looked forward to seeing how McLean brought his script to the big screen.
Director Greg McLean used several actors from the Marvel Cinematic Universe: Gregg Henry (Guardians of the Galaxy (2014)), Michael Rooker (Guardians of the Galaxy (2014) and Guardians of the Galaxy Vol. 2 (2017)), Sean Gunn (Guardians of the Galaxy (2014) and Guardians of the Galaxy Vol. 2 (2017)), and David Dastmalchian (Ant-Man (2015)).
John Gallagher, Jr. (Mike) appeared in 10 Cloverfield Lane (2016) with John Goodman. Abraham Benrubi (Chet) played a younger version (in a flashback) of John Goodman's character (Dan Connor) in a Halloween episode of Roseanne (1988).
The trivia items below may give away important plot points.
A similar experiment testing human behavior was conducted in 1963 by psychologist Dr. Stanley Milgram. The experiment was to focus on the conflict between obedience to authority and personal conscience. One of his concluding remarks stated, "Stark authority was pitted against the subjects strongest moral imperatives against hurting others, and, with the subjects ears ringing with the screams of the victims, authority won more often than not. The extreme willingness of adults to go to almost any lengths on the command of an authority constitutes the chief finding of the study and the fact most urgently demanding explanation."
During the final fight between Mike and Barry, the employee training video states that Belko has locations in over 40 countries around the world. In the final shot of the film, we see monitors of other Belko Experiments, Stage One coming to similar ends. There are 165 monitors visible, and more were being added when the screen cut to black.