Samuel L. Jackson said of working with James McAvoy on the movie, "As good as I like to think I am or what I do and how I do it, watching somebody transform characters in front of your eyes and have an argument with four different people is pretty amazing."
Director M. Night Shyamalan was able to incorporate unseen footage from Unbreakable (2000) into this film, for flashback scenes involving the younger versions of David and Joseph.
Although Disney owns the rights to Unbreakable (2000), director M. Night Shyamalan retained the rights to any potential sequels, so that the studio could not make one without his involvement. Such was his desire for creative control that he co-financed this film by mortgaging his house.
Director M. Night Shyamalan said that the original cut of the film had a run time of nearly three and a half hours. He "trimmed it up a bit" by cutting three of Kevin Crumb's 23 personalities out of the film.
The original script for Unbreakable (2000) included Kevin as an emerging villain for David to face against, but director M. Night Shyamalan could never make it work within the confines of a single movie. Thus, Kevin ended up being Split (2016) off into his own movie, with this film as the culmination of the original idea.
M. Night Shyamalan: Jai, the man talking to Joseph in the security shop. Shyamalan played the same character in Split (2016), and in this film, he confirms that he was also the Stadium Drug Dealer briefly interrogated by David Dunn in Unbreakable (2000).