The Charlies Angels star was speaking at a Venice Film Festival press conference on Friday ahead of the much anticipated world premiere of Pablo Larrain’s Spencer, in which she portrays the former royal.
“There are some people that are endowed with an undeniable penetrating energy,” the actor said.
“I think the idea of somebody being so desperate for connection and somebody who’s able to make other people feel so good, feeling so bad on the inside, and being so generous with her energy.
“I look at her, the pictures and fleeting video clips, and I feel the ground shakes and you don’t know what’s going to happen,
She added: “I just think that we haven’t had very many of those people throughout history. She really sticks out as a sparkly house on fire.”
In response to a question about Diana’s legendary style, The Happiest Season actor replied that it “didn’t really matter what she was wearing. She was someone who used clothes as armour but was so constantly available and visible.
“She couldn’t hide, she wore her heart on her sleeve and that, to me, is the coolest thing she did.”
The film, which portrays Diana over a weekend during Christmas at Sandringham where she chooses to end her marriage to Charles is expected to cause controversy for its depiction of a swearing, masturbating princess.
Stewart told reporters that “there’s nothing salacious about our intention”, however.
She said: “The really sad thing about her is that as normal and casual and disarming in her air [as she was], immediately she also felt so isolated and lonely.
“She made everyone else feel accompanied and bolstered by this light and all she wanted was to have it back.”
Spencer also stars The Shape of Water actor Sally Hawkins, veteran British actor Timothy Spall, and Poldark villain Jack Farthing.
On her role as the ‘People’s Princess’, Stewart said: “Everyone feels like they know her. That’s what’s beautiful about her, that she’s accessible. You feel like you’re friends with her, like she was your mother.
“But ironically she was the most unknowable person… In the imagining of these three days we wanted that to really come to a head.”
Spencer is due for UK release on 5 November.