Beam Me Up, Scotty!: Some people misremember the final line of the movie as "Mr. Glass... they called him Mr. Glass" or some variation, probably because it would serve as a call back to Unbreakable.
Career Resurrection: Fulfilling the promise of The Visit's relative critical and financial success, Split, following about a decade of films that were largely commercial and/or critical disappointments, proved to be one of the most acclaimed and financially successful films that M. Night Shyamalan has ever produced, and certainly the most successful since at least Signs.
Dawson Casting: All three of the teenage girls are played by actresses in their early 20s. Sort of mandatory because of the gradually diminishing clothing, but otherwise they do pass well as teenagers.
Not to be outdone, Anna Taylor-Joy lost about 15 lbs from her already slender frame to play a deeply traumatized teenager.
Playing Against Type: James McAvoy plays several distinct characters, none of which is all too similar to the tortured pretty boys he's often cast as.
Refitted for Sequel: Kevin was originally intended as a main antagonist for Unbreakable, but Shyamalan couldn't figure out a way to make both work in the same movie.
Sleeper Hit: Released in the Dump Month of January, starring an actor who has had trouble as a leading man (in spite of his talent), and being directed, produced, and written by a creator who was largely reduced to a laughingstock in spite of a solid start to his career, the movie wasn't expected to do much better than the standard low-budget horror flick. But a surprising amount of pre-release hype with good word of mouth by test audiences and critics, most of the people who saw the film deemed it to be surprisingly good. The film managed to double its opening weekend projections and held on to the #1 spot for several weeks, due to the low budget becoming one of the most profitable movies that Shyamalan ever directed.
Throw It In!: McAvoy said that trying to get in characters for the video journals sequence he attempted to psych himself up by punching the wall, and ended up breaking a knuckle. He tried to roll with it and use the pain to improve his performance, and he didn't go to the hospital for three days. In an aversion, Shymalan said that it produced some really interesting footage (including McAvoy's face turning white) but they went with some other takes.
Joaquin Phoenix was originally cast in the lead role, but dropped out.
The movie was originally going to be a part of the trilogy that Devil was supposed to start, but the planned sequel never got off the ground. Had the film been made as a part of that Stillborn Franchise as originally intended, it still would have been a sequel to Unbreakable.
Kevin was originally intended for Unbreakable, but Shyamalan couldn't figure out how to work him in so he was replaced by the Orange Man.
, Anya Taylor-Joy says that the dance tells a story of Hedwig going to the club, getting challenged to a dance battle, being attacked and bitten by a zombie, then turning into one. It Makes Just as Much Sense in Context.