In a 2024 interview with The Playlist, Alfonso Cuarón spoke about how the 2 lead parts were cast: "When I was writing the screenplay, I thought of Cate Blanchett. I could only see Cate while I was writing. Luckily, Cate got involved in the project very early on. I finished the script and sent it to her. She agreed to do it, and then she became my creative partner. She was part of it, involved in every creative decision we made, all the way through the end of the show, including casting.
She was the one who called me once we were discussing who could play Steven. She called and said, 'I'm just walking out of a dinner, and we were laughing and talking about Un pesce di nome Wanda (1988)' And I instantly said, 'Wait, do you mean Kevin Kline for the part?' She said, 'Yes, Kevin,' and that was how the idea of Kevin came through. So I contacted Kevin, and he was kind of a bit surprised, intrigued, and daunted, but he accepted it. Remember something: Kevin is an American actor, and he was playing a very specific British man, right?"
The exceptionally talented Russian Blue cat at the Ravenscroft's house improvised all its action and dialog. Despite that, it's not credited.
Based on the book by Renée Knight.