Deadline's Pete Hammond praised Stewart's performance, calling the film a "mesmerizing portrayal" of the late royal. The Hollywood Reporter's David Rooney said that the actress' "finely detailed work on the accent and mannerisms is impeccable" and that she "has seldom been more magnetic, or more heartbreakingly fragile."
Meanwhile, the New York Times' Kyle Buchanan called her casting as a "meta stroke of genius." After all, the actress certainly understands the scrutiny and pressure Diana faced.
"If Diana doesn't always want to come out of her room, you can imagine that Stewart has felt those feelings, too," he wrote. "Whether she plays the game or not, there's no real way to win."
Others were not as impressed by Spencer. IndieWire's Ben Croll gave the film a B- grade, called it "a sort of haunted house chamber piece."
The Wrap's Jason Solomons wrote that the film "unfurls in a torrent of ideas and madness, some of it brilliant, some of it quite silly." Still, he said, Spencer "has the potential to become a gothic, almost camp classic."
The movie focuses on the Princess of Wales' vacation with husband Prince Charles, their sons Prince William and Prince Harry and the rest of the royal family at Queen Elizabeth II's Sandringham estate over the Christmas holiday in 1991. During the visit, she decides to end her turbulent marriage.
Stewart prepared extensively to take on such an iconic role in the film, named after Diana's maiden name, and underwent a temporarily transformation. In addition to sporting the late royal's signature short blond hairstyle, the actress also puts on a posh British accent for the film.
"The accent is intimidating as all hell because people know that voice, and it's so, so distinct and particular," Stewart told InStyle magazine in 2020. "I'm working on it now and already have my dialect coach."
Spencer, which also stars Sally Hawkins, Sean Harris, Timothy Spall, Once Upon a Time alum Amy Manson and Poldark's Jack Farthing, is set for a wide release on Nov. 5.