M. Night Shyamalan says his famous plot twists not calculated
M Night Shyamalan has always been fascinated by multiple personality disorders since his time as a New York University student when he took courses on the subject of dissociative identity disorder (DID).
"The disorder is absolutely fascinating. It occurs only to people that have been traumatised repeatedly between the ages of one and five.
"If that same trauma has happened to you after that, you will not have this disorder. You will have something else.
"It is while the brain is developing when you have these experiences that it happens. Just absolutely amazing thing, what the brain is capable of in terms of survival."
For his research, he read a great deal about the most documented cases.
To inform his supernatural tale, Shyamalan spoke with psychiatrists in the field and gained practical knowledge about how therapists would conduct themselves in sessions with such patients.
That research fleshed out the characters who became Kevin and Dr Fletcher, the therapist who treats him, played by Betty Buckley.
In the process, Shyamalan had to grapple with interesting concepts.
"If people with DID are not like us, does that mean that they're less than us? Do they have abilities that we don't have? Do they have a point of view that is maybe richer than ours or more grounded in a way that ours aren't?" he asks.
"This is a very powerful subject for me. It is something that I think about a lot."
Shyamalan is famous for the twists in his movies, but he doesn't look at it that way.
"I think of it more as plot coming out of character. So if you are the bad person, I don't necessarily have to tell everyone that right off.
"I can convey you are the kindest person for 98 minutes, and then we find out the other. That's it. That would be considered a twist by people.
"But really, it is just about character and a way of telling that character's story. I don't really think of it like, I am going to do my big dance move now."
Always having written original material, he has stayed away from sequels but says he is actually considering one for this movie.
"I can't promise it because I haven't been very good at going into the room and writing something that isn't new. That is not how my engine works. But that is my intention.