The Spencer actress says playing the late royal was a “daunting and exceptionally harrowing experience” and getting inside “that head” had filled her with a “spooky kind of power”.
The star also spoke of her fears of “mucking up” Diana’s British accent, and her hopes to become a mother herself one day in the future.
Promotional clips for Spencer see Kristen’s Diana breaking down in tears in front of her young son Prince William, as she struggles to cope with her eating disorder in the early nineties.
As well as being a mother to the future King of England, Diana was also a devoted mum to his brother, Prince Harry.
Speaking with MailOnline, Kirsten confessed she felt the Princess was at her most “untouchable” when she was with her two young sons, saying “She’s like this feral animal you wouldn’t dare accost!”
Unlike Diana, Kristen points out she’s “not a mum yet” but it is something she is keen to change.
After joking with the interviewer, claiming she’s pregnant, the actress admits she would like to start a family one day soon.
“‘No! I’m not…I’m really not!!” the star retracted, after saying she was pregnant.
“I’m not pregnant, yet. But one day…absolutely! One day I would absolutely love to have a family.”
Kirsten also revealed the extensive research she carried out in order to take on the role of the historical figure who still grabs headlines 24 years after her death in 1997.
The star read “every memoir” about the late mum-of-two and spoke with people who worked in the houses she was living with.
American-born Kirsten did have some concerns about perfecting Diana’s classic English accent.
“I definitely knew that the chances of ‘mucking this up’, as a Brit might say, were far greater than doing a ‘good job’,” she confessed in her interview.
“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.”
Spencer, which is released in cinemas next month, has attracted some criticism following its premiere earlier this month.
Royal experts have accused the new movie of stripping Princess Diana of her dignity and dubbed it “cruel” and “unnecessary”.
Ingrid Seward, editor in chief of Majesty Magazine, said: “It is really cruel to portray her like this. It is totally unnecessary.
“William and Harry will be very angry and hurt about this. They will find it horrible that their mother is being portrayed in such a huge movie this way.”
Spencer gives viewers an dramatised account of Diana’s life shortly after her marriage to Prince Charles has collapsed.
Set in 1991, the film documents the People’s Princess spending three days with the royal family at Sandringham House, where she decides to finally end her turbulent marriage to the future King.
Some critical commentators have taken issue with the late royal’s eating disorders being covered on screen.
For help and support on eating disorders contact Beat Eating Disorders on 0808 801 0677