As Natalie, a devoted spokeswoman for the shadowy Lumon Industries on “Severance,” Sydney Cole Alexander has an exacting morning routine. It starts with that tightly coiled updo.

Ben Stiller, one of the Apple TV+ drama’s executive producers, “has a reputation for being very particular about hair,” Alexander said. After she was cast in the series, which follows workers who sign up for a procedure that bifurcates their personal and professional lives, she sat for a camera test. Stiller had asked her to repurpose the freeze-dried smile she flashed in a Crest commercial she’d booked the same day as “Severance,” and she encouraged him to let Natalie, the headset-wearing “conduit to the gods,” as she puts it, go blonder.