M. Night Shyamalan is known for thrilling shock twists in his films, but the biggest twist of all happened in real life and even took some of Hollywood's finest by surprise.

Despite being an outspoken anti-sequelist, the director is about to release Glass, the final instalment in his Unbreakable trilogy.

Not only that, but he's intertwining the stories of the three unlikely superhumans introduced in Unbreakable and Split into one crossover film in which Samuel L. Jackson, Bruce Willis and James McAvoy will share the spotlight.

That twist was only revealed in the final scene of Split, which was shot with a separate, smaller crew and kept secret until the film's release.

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Even Samuel L. Jackson didn't find out until he saw the scene in a private screening, and Shyamalan can still remember the actor's shock.

"He gets in the car and goes, 'but what does that mean? What does that mean?' And I tell him: 'I'm making a final movie where you guys all meet.'"

Shyamalan says he had to perform mental gymnastics to convince himself to produce a follow-up, but once he did he knew he needed the stars' individual skill sets to make it work.

"I knew I needed to show Sam's conviction and swagger," he explains. "For Bruce, I wanted people to see what I see: his humanity, his father side. And with James, it was all the things he can do physically and the complexities that drive him as an actor."

While fans only saw McAvoy as the murderous "Beast" in the final moments of Split, Glass audiences will see him activate beast-mode for the majority of his scenes. It was a big shift for McAvoy. Not only did the actor gain 10kg in mere weeks, but the mental toll of taking on such a part was also a concern.

The Scot, however, is no stranger to high-intensity roles, and describes playing Kevin Wendell Crumb as "a dream".

"I've done other movies that got weirdly under my skin, where I just felt uncomfortable and ill at ease...But this one weirdly not. The Beast is doing some pretty horrific things, but he's doing it because he has this humongous euphoria for the broken. He feels them."

For all of Split's resounding praise, several dissenting voices emerged criticising the film's portrayal of mental health. McAvoy is aware of the reaction, though it didn't sway him from participating in Glass.

"I have to do my work and research on people who live with that condition, both successfully and unsuccessfully...not saying, 'We can't play someone doing bad things if they've got DID [dissociative identity disorder]' on some weird positive discrimination. I need to pay them respect by doing my job as best I can and not judging the work."

Samuel L. Jackson is also passionately committed to bringing comic-book-theorist-turned-terrorist Mr Glass to life. The intensity with which Glass has followed David Dunn's career as a vigilante always intrigued Jackson, and as an industry veteran, the actor claims his career's mission is to sign on to projects that he finds entertaining.

"People will always want to see movies," he says, "always want to sit in a dark room and experience it with a whole lot of other people...so for me, it's not about accomplishing anything, it's about continuing to tell stories. Now there's the security in that I'm not worried how a project is going to perform when I take it."

That said, his self-assurance doesn't stop him from coming after online critics who oppose his creative choices, "I just go and reply to comments with 'F*** you' or 'Actually, your mother told me to take this role'."

Glass brings three films together into one universe. Photo / Universal Pictures
Glass brings three films together into one universe. Photo / Universal Pictures

While Unbreakable has garnered a cult following as one of the best super-villain origin stories (the fact that deleted scenes from the film will appear in Glass has only added to the hype), it received mixed reviews upon its release in 2000.

Shyamalan was fresh off his smash-hit The Sixth Sense ("He's definitely not as arrogant as he once was," Jackson says of working with the director the second-time around), and the response threw him.

"I remember thinking, well, maybe people just aren't into comic book movies," Shyamalan recalls. After witnessing superhero blockbusters run amok as well as Split's critical and commercial success, he now laughs at the sentiment.

Now he knows what works, Shyamalan is looking to expand on it, with the hopes his signature style will become as instantly recognisable to audiences as the symmetry of Wes Anderson or the gore of Quentin Tarantino.

He dreams of being the filmic answer to Agatha Christie.

From the opening credits to the last sequence, you'll now know an M. Night Shyamalan film...and you can bet Glass is no exception. But that "relationship clarity" comes at a big sacrifice, but in another surprising twist, it's one that Shyamalan is finally willing to make.

"For a long time I wasn't willing to commit to making thrillers forever, but I am now," he says.

"That's what I want. That's the only way I can do what I'm hoping to do, and I think the audience will get that."

LOWDOWN:
What: New M. Night Shyamalan film Glass
Who: Bruce Willis, Samuel L Jackson, James McAvoy, Sarah Paulson
When: In cinemas now