AceShowbiz - Kristen Stewart felt "disloyal" playing Princess Diana because she doesn't have children of her own.
The actress stars as the late royal in Spencer - which focuses on three days at Sandringham House in Norfolk, England the Christmas before the princess split from Prince Charles.
Kristen felt her alter ego was at her most "untouchable" when she was with her sons, Princes William and Harry, and the actress feared she wasn't "authentic" enough in the role because she hasn't experienced a maternal connection in her own life.
She added, "She's like this feral animal you wouldn't dare accost... (I regretted that) I'm not a mum yet. I need that to feel authentic. It's the one part of playing her that I felt disloyal about."
The movie has had a mixed reaction from critics and Kristen always knew it would be a divisive subject, but she ultimately felt it was worth the risk.
She told the Daily Mail newspaper columnist Baz Bamigboye, "I definitely knew that the chances of 'mucking this up,' as a Brit might say, were far greater than doing a 'good job.' "
"But I thought it was worth it. I've been working as an actor for a long time and it's really rare to find things that make you feel this way."
The actress had been prepared for the "daunting and exceptionally harrowing experience" of recreating one of the lowest periods in the life of Diana - who died in a car crash in 1997 - but she wasn't ready for how much "joy" she would feel.
"Diana just really did have this overriding, disarming, beautiful energy," she added. "(The decisions she was making were) earth-shattering. (But) inhabiting her made me feel taller, and made me feel I could dance. I felt her power in a f**king beautiful way."
The American star spent six months researching the role and a lot of time focusing on Diana's "buoyant" accent.
"When I'm talking, I barely open my mouth, and everything comes to an end fairly abruptly," she said.
But on occasion, if she felt "self-protective or tight or emotional" on set, Kristen lapsed back into her natural accent and would end up walking around shouting "Argh! Argh!" in order to get her voice back on track, much to the shock of the crew and her castmates.
Despite her own global fame, the "Twilight Saga" star insisted it's nothing compared to what Diana experienced.
She said, "I mean, her world opened up to the entire planet."