From Psycho (1960) to Fight Club (1999), the notion of multiple personalities inhabiting one person has been fertile ground for drama, not to mention creating the sort of once-in-a-lifetime, awards-baiting role that many actors live for.
Now you can add director M. Night Shyamalan and actor James McAvoy to the list of film-makers and performers entranced by the idea.
Shyamalan, 46, has written and directed a new film, Split, which opens in Singapore tomorrow. It stars McAvoy as a man with multiple personalities, Kevin, who kidnaps three teenage girls.
Speaking to The Straits Times in Los Angeles recently, Shyamalan says he spent two decades gestating a screenplay about dissociative identity disorder (DID), the mysterious mental condition formerly known as multiple personality disorder.
The director made his name with the supernatural thriller The Sixth Sense (1999) and continued to work in the genre with Signs (2002) and The Village (2015) but, for this film, he was inspired by scientific research on the condition.
You assume you're watching a kidnapping movie and then that changes; you think you're watching a supernatural or science movie and that changes. Then the genre changes (again) at the end.
DIRECTOR M. NIGHT SHYAMALAN on Split
It has intrigued him since the 1990s, when he heard that Titanic director James Cameron was trying to make a movie about Billy Milligan, the first man to successfully use the disorder as a legal defence.
"I've loved the subject for a long time. I remember I heard that James Cameron was going to make a movie about DID, based on the book The Minds Of Billy Milligan. And I was, like, 'Oh my god, that's going to be amazing. I want to grow up and make that movie.'"
Cameron eventually abandoned the project, but his Titanic star Leonardo DiCaprio is now developing his own adaptation of the Milligan story, a project the actor has reportedly been pursuing for decades.
Shyamalan has been similarly obsessed: He first began writing scenes for Split 15 years ago, using his research on the controversial disorder that many mental-health professionals do not believe is real, much less agree on how to diagnose or treat.
"It's a very controversial disorder, but I believe in it," the film-maker says. "Those who have it constantly cover it up - they don't want to be outed because they know you don't see them as they are, so it's super fascinating and poignant."
Depending on what personality is dominant when certain tests are run on someone with DID, "one of the personalities can have diabetes or one of them can have high cholesterol and the other ones do not", he says. "It's unbelievable."
Shyamalan believes such phenomena lie on the same spectrum as the well-known placebo effect, which reports that a significant percentage of people get better if given sugar pills instead of drugs.
This suggests that mental states can have real physiological effects, he says.
"It's a daily occurrence that happens in every hospital everywhere around the world - every doctor and scientist knows about it, but they don't quite flip out about it like I do. I'm the one who's going, 'Don't you find that incredible? They're curing themselves.'"
He knew there would not be many performers who could pull off the role of Kevin, which would require an actor to convey nine of the character's 24 distinct personalities using changes to his voice and mannerisms alone.
"Who could do it and have you not think it was silly?"
Top of his list was McAvoy, the Scottish star of the X-Men films (2011-2016) and dramas such as The Last King Of Scotland (2006) and Atonement (2007).
The 37-year-old actor threw himself into the role by imagining what led to the creation of each of Kevin's personalities, or "alters", which include a devout older woman named Patricia and a 10-year- old boy named Hedwig.
McAvoy says: "I tried to figure out what exact moment or event in Kevin's life caused each and every alter to become necessary."
Like Shyamalan, he dived deep into the research and testimonials on DID.
"Something I found from my research is that keeping a diary is useful to someone with DID, just to get through their day-to-day life and, logistically, corral so many people who are operating independently and can't communicate with one another sometimes."
Today, many DID patients use video diaries "to communicate with alters and also to tell the world, 'I know you think we don't exist and think we're attention-seeking, but here we are, we're actually living it'.
"I found that a really useful tool because people are speaking so much from the heart in their video diaries. I found that very illuminating," says the actor, who has a five-year-old son with his ex-wife, actress Anne-Marie Duff, 46.
The other stars of the movie are the young girls kidnapped by Kevin, played by Anya Taylor-Joy, Haley Lu Richardson and Jessica Sula.
Split also highlights similarities between the girls and their abductor. Like Kevin, Taylor-Joy's character, Casey, is a social misfit - something Shyamalan notes has become a "common theme throughout my movies - the marginalised becoming empowered and realising they're stronger than they think".
At the same time, he acknowledges that his stories are getting "darker" and "more perverse", often marrying horror with a twisted sense of humour.
This could be seen in The Visit (2015), his dark tale of a group of children visiting grandparents who start behaving oddly, as well as in Split, which features many moments of offbeat levity amid the horror.
The director says he was going for a tone of "Lynchian, weird humour that's disturbing at the same time", so the viewer is unsure whether to laugh or be creeped out.
"It's an unusual tone, it's hard to hit and control, and I'm okay with that inappropriate perverseness. I'm not sure if 10 or 15 years ago, I'd have written a kid getting smothered with a diaper, as I did in The Visit, or the things that go on in Split," says Shyamalan, who married his college sweetheart Bhavna Vaswani, a psychologist, and has three daughters aged 12 to 20.
"But I also feel okay doing that because I feel like when making smaller movies, I'm allowed to push the boundaries a bit and be more independent and aggressive."
Split was made with a modest US$5 million (S$7.1 million) by Blumhouse, a production house known for low-budget horror flicks.
Like some of his previous work, it will blur the boundaries between genres and, in that sense, have a bit of a split personality too.
"You assume you're watching a kidnapping movie and then that changes; you think you're watching a supernatural or science movie and that changes. Then the genre changes (again) at the end," Shyamalan says.
"I'm drawn to that. It causes a great reaction among audiences."
•Split opens in Singapore tomorrow.