If an actor’s work is to be transformative, beyond reading lines and playing pretend, then the preparation Kristen Stewart underwent to play Princess Diana in “Spencer” was worth the effort. In fact, she was so overtaken with the gravity of the job that it had physical ramifications. “I couldn’t open my mouth for two weeks before we started shooting,” Stewart told the BBC. “I had TMJ to the point where I was like, completely locked up. I was like, ‘Huh, I guess I’m really nervous – I was really tripping out until we started.”

Ultimately, the role burrowed itself deep into Stewart’s psyche. “It felt so f***ing spiritual and so spooky, and I was so overwhelmed by the feeling of her. And I don’t mean like ghosty stuff. I just mean everything she’s made me think and feel, and it all just kind of happened in a moment,” Stewart told Entertainment Weekly, regarding a scene in which she descended a stairwell fully in character as Diana.

Stewart further told the Los Angeles Times that Diana’s death repeatedly upset her. “She felt so alive to me when I was making this movie,” Stewart said. “But there were moments where my body and mind would forget she was dead.” Two or three times each week, during the filming of “Spencer,” Stewart said, she “would just fully break down about the fact that she had died. I just could not come to terms with it.”

In addition to providing a lot of the signature sounds that make Radiohead one of the most experimental and future-forward bands in music, guitarist Jonny Greenwood has a sideline as a film composer. He’s writer-director Paul Thomas Anderson’s go-to musician, providing the soundtracks for “There Will Be Blood,” “The Master,” “Inherent Vice,” and “Phantom Thread.” In 2021, Greenwood’s scores underlie and enhance three movies with major awards buzz: “The Power of the Dog,” Anderson’s “Licorice Pizza,” and “Spencer.”

Greenwood struggled with scoring “Spencer,” and he tried hard not to go with classical-style orchestral music, as that’s what’s overwhelmingly used in biopics and movies about the British crown. “I watched a few royal films, which were full of sweeping shots of Buckingham Palace, with fanfare horns and tinkling harpsichords on top,” Greenwood told NME. “I wanted instead to emphasize how chaotic and colorful Princess Diana was, in amongst all that baroque tradition. It’s what the film does too.”

The way Greenwood got the music he wanted: He wrote the score “in that regular royal style,” and when it came time to record, he switched out the musicians stationed to traditional instruments — kettle drums, pipe organs, harpsichords — with improvisational jazz musicians. “The key was to still sound vaguely baroque, while leaving enough space for true anarchy and chaos.”

“Spencer” is rooted in truth, and it’s based on real events that happened to a real and extremely famous individual, Princess Diana of the British royal family, but “Spencer” is not a biopic, or a film that tells the entire life story of its subject. “Spencer” speculates on how the events of a weekend in late 1991 played out just before Diana announced her intention to separate from Prince Charles and officially leave royal life. The entire movie takes place over those few dozen hours, and it’s largely the invention of screenwriter Steven Knight and the execution of director Pablo Larrain, who happens to specialize in such nontraditional, conjecture-based, not-quite-biopics of major public figures.

The long sequences in “Spencer” of a deeply emotionally troubled Diana (Kristen Stewart) roaming around Sandringham House, lost in thought and anguish, is reminiscent of “Jackie,” Larrain’s 2016 film in which Natalie Portman portrayed First Lady Jacqueline Kennedy, numb and deeply grieving days after the assassination of her husband, President John Kennedy. Also in 2016, Larrain directed “Neruda,” a biographical film about Nobel Prize-winning poet Pablo Neruda, but only and specifically the part of his life when he became a pariah on the lam in Chile for his Communist views.

The plot arc of “Spencer” concerns Princess Diana coming to the tough decision to separate Prince Charles, her husband and father of her two children, William and Harry. As Diana comes to figure out how she can leave the royal life she’s come to loathe, “Spencer” engages in many graphically dark and urgent sequences, including the princess hallucinating her own death, contemplating choking to death on a pearl necklace, throwing herself down a flight of stairs, and purposely vomiting (Diana suffered from bulimia, per Vogue).

From a filmmaking perspective, those scenes speculatively demonstrate Diana’s inner agony, but according to some royal experts and critics, those parts of “Spencer” were uncalled for. “It is really cruel to portray her like this. It is totally unnecessary,” Majesty Magazine editor Ingrid Seward said to The Sun. “William and Harry will be very angry and hurt about this.” Royal watcher Penny Juror concurred, calling the scenes in question “unnecessarily gratuitous” and even questioning if the film is “factually incorrect” as it pertains to such events.