The man who introduced the notion of seeing dead people in The Sixth Sense explores Dissociative Identity Disorder in Split, with James McAvoy exhibiting 24 different personalities.

Writer-director M. Night Shyamalan originally wrote the Split script with Joaquin Phoenix in mind, having previously worked together on The Village and Signs. But when Phoenix dropped out, James McAvoy stepped in – no stranger to multiple personality disorder, having already made a trial run in Filth.

A stickler for homework, Shyamalan, 46, has done much research into Dissociative Identity Disorder, otherwise known as Multiple Personality Disorder.

DID is still the black sheep of the psychiatric world, many believing that patients invent their “multiples” to dodge prison sentences or that therapists actually “push” the DID diagnosis on fragile patients who may be suffering from other psychiatric disorders.

When STACK meets with the director in  Los Angeles, his eyes light up at the sheer numbers. “Oh there are as many as like 60 personalities [that have] been diagnosed in one patient. Tons! I mean even Sybil Dorset had 16 and the famous ones, like Billy Milligan, was 22, or something like that,” he says, referencing The Minds of Billy Milligan, a film treatment to which Leonardo DiCaprio has long been attached, while the former’s story was told in the 1976 mini-series Sybil, starring Sally Field.

For Shyamalan, however, the 1957 film The Three Faces of Eve, starring Joanne Woodward in an Oscar-winning performance, really spoke to him.

“I found it very moving. I just find this psychological disorder absolutely moving and tragic, and amazing too. Especially the science of it. Pretty much everything I do in the movie, with one exception, is true.

“DID is a disorder that only happens to individuals who were consistently sexually or physically abused between the ages of one and five years old, which is when the brain is still developing and the brain actually starts different synapses, and starts a different area, and goes ‘We can’t deal with the fact that our uncle or mother, or whatever it is, is doing this, so we’re going to create a whole other existence’. And then once it learns to do that during those years, it does it forever; it keeps on splintering over the course of time, so that these different personalities all become options for them,” explains Shyamalan, who worked with some of the top researchers in the DID field; genuinely compassionate of those afflicted with the disorder.

With all his research into DID, it was little wonder he felt anxious once the deal with Phoenix failed and he began recasting the lead. “But sometimes it’s like the right human being walks into your life at the right time. James was actually very aware of this subject, so I wasn’t starting from zero. Not many people can do empathy, comedy, and the physicality all at once. That was what scared me. But he was able to do it all.”

Split also features The Witch‘s Anya Taylor-Joy in a key role; the actress was yet to hit the zeitgeist when she first auditioned. “I was unfamiliar with her but just found her mesmerising,” says Shyamalan. “She’s just an odd soul. She can’t be normal, and she’s obviously very beautiful, but she has the energy of this odd, ethereal person with her china doll-like face. And there’s a darkness there underneath, which I got to bring out.”

Split is in cinemas on January 26.