James McAvoy plays 23 roles in new movie Split

In M Night Shyamalan's latest horror film Split, James McAvoy plays a man with dissociative identity disorder (DID).

He has multiple personalities, 23 to be exact.

To McAvoy, that acting challenge was fun because "I'm mental".

"It was a low-budget movie on a short schedule, so I didn't have that luxury of doing one character a day - sometimes I was doing four, five characters a day. But I enjoy acting.

"Getting to do nine times the amount of what I usually do is nine times the work, but it was nine times the fun."

In Split, which opens here tomorrow, three teenage girls are kidnapped by McAvoy's character Kevin and he keeps them locked up in a bunker.

Every time he visits them, he frightens and disorients them by displaying a different personality and the girls have to work out which personality to appeal to for them to escape.

This has to be done before the arrival of the final personality, the Beast, that controls all the other ones.

So which personality is with us today at the Four Seasons Hotel in Beverly Hills?

It was a low-budget movie on a short schedule, so I didn’t have that luxury of doing one character a day — sometimes I was doing four, five characters a day. James McAvoy

"The jet-lagged tired one," McAvoy answers, having just flown in from London.

The 37-year-old Scot talks about how Shyamalan came to him with the role.

"Joaquin Phoenix was doing it and then he pulled out. And then I met Night at a party in San Diego at Comic-Con and he was like 'hey, you're bald!' And I was like, 'yeah, good chat, see ya'," he says, laughing.

"Then he called up a couple of weeks later and he was like 'Listen, I've got this script'.

"I read it and I was very, very excited. There was a part of me that thought, it's either going to fail spectacularly or be really, really good - the margin of error will really turn on a knife's edge. But my hesitation lasted for about two seconds."

Shyamalan impressed him by being a very prepared director.

"He knows every word back to front, he knows every beat forward and backward.

"Night doesn't do storyboards. He just draws it on a piece of paper.

"And of course he will improvise with it if he needs to but, by and large, he is filming what he envisaged and the same with the dialogue as well.

"I am an actor who makes stuff up a lot and I am keen on doing that, but Night was like 'no, I sweat blood over the script, so let's go with what I wrote'.

"So even if you are thinking 'I am not sure about this moment', you know he is sure about this moment. It gives you even more bravery to go into this movie that demanded a brave performance.

"There is a real family vibe on set as well. And I would work with him again if he wanted me to because it was a great experience."

McAvoy found it important to bond with the young actresses - Anya Taylor-Joy, Haley Lu Richardson, Jessica Sula - before shooting the difficult scenes.

"I thought, I am going to reach out to them. We have got to do some fairly invasive stuff together and I thought it would be better if the girls felt safe.

"Because I know as an actor I can act better and I can tap into fear better if I feel safe.

"So what then happened was that the girls and I got on like a house on fire, and they proved themselves to be three of the funniest people I have ever worked with in my life."

McAvoy did not meet people with DID, but he did do a lot of research.

"What was very useful was reading and watching a few documentaries.

"The Internet and YouTube are fantastic tools for people with DID, because they can actually have a conversation with each other on video.

"There are quite a lot of people with DID who actually blog what it's like to live with DID day to day. And that was so fascinating and valuable for me."