The 31-year-old actress plays the late Princess of Wales in the new film, 'Spencer.'
"The whole thing about watching her, within that space when her children are close to her, it's a nucleus. It's a three-headed animal, sort of an unstoppable beast," Stewart told ET's Lauren Zima of the late Princess of Wales' relationship with sons Prince William and Prince Harry. "...You don't want to mess with moms, you just don't do that. I think [Diana] feels so embodied and so unconditionally herself when when she is with her kids. In all of the research, that I was like, 'Man, that is what we are protecting here.'"
Despite not personally having experience with motherhood, Stewart related to Diana on other levels.
"I mean, I'm a kid of a parent," she said. “I do know what it feels like, that exchange. I can’t say with certainty that I know what it feels like to be a mother. But I have, you know, I have all the natural feelings. I am human.”
Director Pablo Larraín jokingly added, “And a puppy!”
"I do love my dog," Stewart emphasized.
Larraín took a strong interest in Diana's universal appeal.
“One of the things that made me very curious is why she affected so many people all around the world? How did she become such a world icon? And what was people, especially women, seeing in her? And that helped me," he explained. "And just talking to people like my mother."
As for Stewart, she said of portraying the princess, "I felt like I absorbed her and then trusted the script."
Spencer hits theaters on Nov. 5.