Rose Glass originally wrote Maud with a more explicit backstory, but removed most of it in the final draft as she found it too similar to Carrie (1976), saying: "In early drafts, the character's backstory was quite different, she had this very extreme religious upbringing, went to Catholic school, all that stuff. But it just felt like a story I'd seen before, and it wasn't one I was particularly interested in retelling."
Maud's eyes are two different colors: one blue/green and one brown. However, the actress Morfydd Clark has only blue eyes.
Originally shot in the taller than normal 1.66:1 aspect ratio, as director Rose Glass wanted more space and headroom around the characters. It was re-framed in post-production to widescreen 2.39:1, with a few shots extended horizontally with VFX.
Director Rose Glass won the IWC Schaffhausen Filmmaker Bursary Award for this, which was presented to her by Danny Boyle.
When Maud/Katie hears God speak during her vision, the language he is speaking is Welsh.