Sophie Turner studied schizophrenia and other mental illnesses before shooting 'X-Men: Dark Phoenix'.

The 21-year-old actress returns to the franchise as psychic Jean Grey in the next outing in the series, and revealed the ''amazing superhero story'' is also very ''personal'' and required a lot of research.

She said: ''Once I got the script through, I realised that not only were we telling this amazing superhero story, but we were also telling a very personal story as well.

''One which required research into mental health issues because that is a big part of what drives the story.

''I mainly drew inspiration from multiple personality disorder and schizophrenia, as it really is about duality, this movie.

''Darkness and light - that's all within her.

''This sense of her being a completely other persona and struggling with that.''

And Sophie admitted the movie has been a huge challenge for her.

She said: ''I kept saying to Simon (Kinberg, director), 'Oh good, that's nearly done. Now I only have, like, seven huge emotional scenes to go. It was a real challenge.''

Simon has stepped behind the camera for the first time, having previously been a writer on the saga, but Sophie insisted he didn't seem inexperienced and praised him as one of the ''best'' directors she's ever worked with.

She told Empire magazine: ''Simon has been the brain behind 'X-Men' for years and it's really exciting to have what feels like one of our own rise up and take hold of the franchise and direct it in the way that he sees it.

''I have to say he's one of the best directors I've ever worked with. He's so passionate and collaborative.''