Inspiration for the movie, real-life multiple-personality Billy Milligan (February 13, 1955 - December 12, 2014), charged with three rapes, was the first person diagnosed with multiple personality disorder to use an insanity defense by reason of that disorder, and also first to be acquitted thus. Milligan had 24 personalities, consisting of 10 Desirables: Billy Milligan, Arthur, Ragen Vadascovinich, Allen, Tommy, Danny, David, Christene, Christopher, and Adalana; and 13 Undesirables: Phil, Kevin, Walter, April, Samuel, Mark, Steve, Lee, Jason, Bobby, Shawn, Martin, and Timothy; and The Teacher, a fusion of all of the other personalities.
James McAvoy broke his knuckle while filming on location in Philadelphia. He continued to perform his role for three days before letting on that he was injured and finally sought attention at a local UrgentCare. As a result he was forced to work with an injury on the set of Atomic Blonde (2017) as well.
Director M. Night Shyamalan appears in the movie as Jai, the fast-food loving concierge of the apartment building where Dr. Fletcher works. They watch the security cameras after Barry leaves the building.
Writer-director M. Night Shyamalan cast James McAvoy after a chance meeting with him at Comic-Con where McAvoy had recently completed X-Men: Apocalypse (2016) and his hair was still little more than scalp stubble. During the course of the conversation with the soft-spoken man, Shyamalan began to realize that he'd found his leading man, and he would be bald, have no makeup, and look just like McAvoy.
On Kevin's computer, there are QuickTime files of his various personalities. The one in the bottom right corner is Mr. Pritchard. This is likely a reference to the character Lionel Pritchard from Signs (2002), also directed by M. Night Shyamalan.
Shyamalan mentioned in a interview that he licensed music from previous films scored by Ennio Morricone and tracked the film with his music, but ultimately thought that it was too old fashioned and hired composer West Dylan Thordson to write an original score.
The last thing Kevin remembered was being on a bus on September 18th, 2014. This is the date which Scotland held a vote to decide if they were going to split from the UK. Kevin is played by James McAvoy who is from Scotland and has been outspoken as pro-independence.
Also contains a spoiler Glass (2019). A young Kevin Wendell Crumb and his abusive mother are speculated widely to be seen briefly in Unbreakable (2000), in the scene at the stadium where David Dunn brushes past a woman and a child and senses child abuse just before he confronts the drug dealer. In this movie is revealed that Kevin himself is a victim of child abuse, with his mother being the abuser, and that his father left on a train. Glass (2019) confirms that Kevin's father died in the train wreck that begins Unbreakable (2000).
In Kevin's final scene, as the camera tilts up the stairs, the music playing is the beginning of the theme to Unbreakable (2000), "Visions". As the scene switches to the diner, where Bruce Willis makes an uncredited appearance as David Dunn, the iconic theme of his character from that movie is heard. Not only does this reveal that Unbreakable and Split occur in the same cinematic universe, it also hints that Kevin may not just be a madman: if David Dunn is considered to be a real-life superhero, this would make Kevin transforming into 'The Beast' a supervillain.
The last time Barry visited Dr. Fletcher, you can see how he opened the front door using Dennis' yellow washcloth and then putting it in his pocket after walking in, showing that it had been Dennis all along, pretending to be Barry.
The movie poster that shows cracked glass is similar to the poster for Unbreakable (2000) and alludes to the fact that both movies are within the same cinematic universe. This has been confirmed by the director of both movies, M. Night Shyamalan. He also stated that he intends to make a third movie with characters from both films. Both movies also have titles that coincide with the main character. In Unbreakable, Bruce Willis' character David Dunn survives a train crash that leaves him unbroken. This movie's main character, Kevin Wendell Crumb, has split personalities.
In Unbreakable (2000), Elijah Price's mother describes a long fingered creature with a massive mane of hair as the "soldier villain", who fights the hero with his hands, just before inadvertently describing her own son, the arch-villain, who battles the hero with his mind. Interestingly enough, the physical description of "The Beast" by his minions is identical to that of the monster battling the superhero in the comic book artwork. This is likely indicating that David Dunn has now found his physical match with The Horde, while his archenemy Elijah is still plotting from behind bars.
Hedwig tells Casey he has a CD player in his room and says "I love Kanye West, he's my man!" Kanye West references M. Night Shyamalan's previous movie within the same universe, Unbreakable (2000) in his song "Through the Wire" in the line "Unbreakable, what you thought they called me Mr. Glass?"
The dialogue about the Beast contains a lot of language that overlaps exactly with dialogue about Mr. Glass. Dennis says the Beast is not a mistake. Likewise, in Unbreakable (2000), Mr. Glass says "I'm not a mistake". The characters in both films use the words "extraordinary" and "human evolution" to describe themselves, and often say "the world will understand now". The description of the Beast given by Dennis also matches the description of the soldier villain described by Elijah's mother: long mane of hair, muscular, fingers twice as long as normal.
There is a scene near the end of the movie showing 23 different toothbrushes for each of Kevin's multiple personalities. Similarly, the closing credits are tiled and appear 24 times, accounting for the newly added Beast personality.
The color pattern of the wardrobe also corresponds to the color pattern in Unbreakable, where purple clothes correspond to super-villains (Mr. Glass wears a purple overcoat and drives a purple car and Dennis, pretending to be Barry, wears a purple coat on his first visit to Dr. Fletcher) and green for heroes (David Dunn wears a green raincoat, Dr. Fletcher a green scarf and when Casey's white shirt is ripped by the Beast, it reveals a green undershirt).
There are clues as to where the girls are being kept and Kevin's employment. 1. Their abduction (and the alleged previous ones) took place at the King of Prussia Mall. There is a zoo near the mall with an underground maintenance area, large enough for the abductions to go undetected. 2. There are various animal statues in several scenes. 3. Hedwig's room contains animal drawings with their names. 4. The rock wall in the first locked room the girls are held in is consistent with the walls used in zoo enclosures. 5. There was mention of a school field trip to Kevin's place of work. Kevin has a variety of clothing - presumably from the 'lost and found' at the zoo. 6. Dr. Fletcher mentioned that "The Beast" was formed because of "your time in this place", the zoo.
The names of the 23 identities are (in order of the computer files): Barry, Jade, Orwell, Kevin, Heinrich, Norma, Goddard, Dennis, Hedwig, Bernice, Patricia, Polly, Luke, Rakel, Felida, Ansel, Jalin, Kat, B.T., Samuel, Mary Reynolds, Ian and Mr. Pritchard. The 24th identity, The Beast, was the last personality to be formed, by a trigger. Only 8 identities are listed in the credits: Dennis, Patricia, Hedwig, The Beast, Kevin Wendell Crumb, Barry, Orwell, Jade.
Like 'superhero' David Dunn from Unbreakable (2000), Casey Cooke's first and last names begin with the same letter, much like comic book characters such as Peter Parker, Bruce Banner, Matt Murdock, Reed Richards, Lois Lane, Lex Luthor, etc.
When Dennis, as Barry, is walking down the spiral staircase from Dr. Fletcher's office the camera focuses on the stained glass window on the ceiling. If you count each piece they add up to 24, possibly foreshadowing that there are actually 24 personalities, not just 23.
Japanese moviegoers were upset to find out M. Night Shyamalan had spoiled the movie himself when he announced the sequel on his Twitter in April 2017, as the movie wouldn't open in the country until May.
James McAvoy played professor Charles Xavier in the X-Men prequel trilogy. Professor X's son in the comics, David Haller/Legion, is a mutant with dissociative identity disorder. Legion had its own on-screen debut in Legion (2017) the very same year "Split" had a wide release.