Ashwin Dev’s review published on Letterboxd:
Spencer is an indelible cinematic exercise in subjectivity crafted by the Larrain-Stewart-Greenwood trio. Very much like a shell-shocked Jacqueline Kennedy in the iconic blood-stained pink dress who was forced to plan her husband's funeral in Jackie, here Diana Spencer, at the verge of a mental breakdown, has to spend three days with the insufferable royal family.
As clichéd as it may sound, Stewart simply disappears into the character, and wanders around the immaculately decorated maze that Larrain has designed for her, and Greenwood uses classical instruments to induce a kind of nausea of feelings and thoughts, if you will. Just listen to his track 'The Pearls'; it starts off as a tingle in the stomache, then slowly creeps its way up, spreads all over your chest, and develops into a full-blown, claustrophobia-induced regurgitation.
If I'm nitpicking, the weakest aspect of the film is its screenplay because it has a tendency to make things a bit too verbal when it comes to some visual metaphors (pheasants) and the Boleyn parallel (the best version of Spencer doesn't really need so many lines of dialogue). To its credit, for something that stays with Diana the entire time, it manages to listen just enough to the family as well. The queen speaks barely two sentences but that one line hits like a condensed description of all four seasons of The Crown; and even Charles comes off as a real person. A lesser film would have implied that they're all bloodsucking monsters and left it at that.
With Spencer, Larrain isn't breaking any new ground; it's exactly the same formula that he used in Jackie but I'd watch a hundred more of these.
PS The Academy: For your consideration, Kristen Stewart for Best Actress in a Leading Role, and Jonny Greenwood for Best Original Score.