Maud's eyes are two different colors: one blue/green and one brown. However, the actress Morfydd Clark has only blue eyes.
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."
Director Rose Glass won the IWC Schaffhausen Filmmaker Bursary Award for this, which was presented to her by Danny Boyle.
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.
When Maud/Katie hears God speak during her vision, the language he is speaking is Welsh.