M. Night Shyamalan's Glass: Everything We Know So Far
8. It's Been In The Making For 20 Years
Split's surprise twist ending may have felt just a little tacked on, but Shyamalan's decision to add that extra scene wasn't some rushed, last-minute choice; he'd been planning it since before Unbreakable even came out.
According to Nerdist, Shyamalan said that "this [McAvoy's Kevin Crumb] was a character that was in the original screenplay [for Unbreakable],” while speaking at AFI Fest in 2016.
“I pulled him out", he continued. "He was throwing off the balance of the piece [by being] too many characters.”
This means that, at one point in time, there wasn't a separate movie for Kevin - originally, he was involved in the Unbreakable franchise since the beginning, but Shyamalan removed him because it made the story too crowded.
This also means that Shyamalan has been thinking about the Split/Unbreakable connection for around twenty years (Unbreakable opened in 2000), meaning that the Kevin versus David Dunn showdown - one we'll see in Glass - has been a long time coming.