Author's Saving Throw: Chris Chibnall said the Doctor would be darker this season, after complaints last season of her being, for the most part, nice and passive. That definitely started this episode, mind-wiping memories, getting called out on not explaining herself and leaving the Master's British Indian incarnation to some angry Nazis.
Deleted Scene: According to Aurora Marion's Instagram, a scene showing Noor Inayat Khan's execution by the Nazis was filmed, but not included in the final cut.
Fatal Method Acting: Jodie Whittaker shared on The Graham Norton Show that she came unsettlingly close to being killed while filming this episode, when a South African sac spider landed on her collarbone, causing her to brush it all the way down her body. Then another crew member picked it up with his bare hand and threw it away, ignoring the locals who were screaming at him not to touch it.
Foiler Footage: When filming the scene where the Master makes the Doctor kneel and say his name, Jodie Whittaker was told to say "Micah" instead so the extras would not know O's real identity. She only said "Master" when filming the close-up, when the extras were no longer in the room.
In Memoriam: The first part is dedicated to long-time series writer Terrance Dicks, who died in 2019. He's even credited as "the Masterful Terrance Dicks". He was the script editor during the entirety of the Third Doctor's era, which introduced the Master to the franchise.
Sacha Dhawan unintentionally spoiled he was playing the Master to former showrunner Steven Moffat while working on Dracula (2020). Moffat had worked with both Dhawan and Chris Chibnall on Doctor Who and asked him how filming was going over dinner. Dhawan said it was going fine, but he was having trouble with techno-babble speech like "Tissue Compression Eliminator". Moffat, a lifelong Who superfan, instantly recognised that as a weapon of the Master's and sussed out that Dhawan had been cast in the role.