'I love her' - Kristen Stewart on portraying Princess Diana

Since its world premiere at the Venice film festival, the movie has been given glowing reviews and specifically Stewart's performance.

Films about prominent royals aren't often well-received. 'Diana' starring Naomi Watts and 'Grace of Monaco' starring Nicole Kidman were mauled by critics.

Stewart admitted that she was scared by the prospect of portraying the late princess.

"I think I have sort of more like relatable and literal fear because I do the walking and the talking of it," Stewart said.

She added "Even if people hated it and it ended up being like a sort of misfire, we tried the best we could. I mean, it was just like this wasn't something I could pass up. I had to give it a shot."

The director, Chilean filmmaker Pablo Larrain ('Jackie'), was unaware that Stewart was frightened of taking the role, saying to Stewart during the interview, "I never felt fear in you and if you had it, you hide it really well because I felt confident because you were also very confident. Otherwise, it maybe creates a domino effect of panic. I guess it could happen to you if I would be super-scared."

Despite the fact that Stewart seems immersed in the character on screen, she revealed there were no rehearsals to help her get into character.

She said "The first time they started rolling, that was the first time that we actually had every element come together and be one."

But that didn't mean Stewart was unprepared. She said "We absorbed her the best that we possibly could, read everything, looked at everything. I emotionally connected with her. I love her. I'm not alone in that. She is loveable. She is very easy to feel protective over."

'Spencer' is set during the Christmas of 1991 at Sandringham Palace. Knowing her marriage to Prince Charles is all but over, Diana struggles to engage with the pomp and ceremony of a royal festive season.

Although the film appears to add new elements to Diana's life, like her driving her own car and refusing to wear her official clothes on the correct day, Stewart admitted they were "purely speculative".

"There are things that we just can't know but we'll be endlessly curious about. The details don't matter," she said.