When the cast of The Diplomat gathered for the final table read of Season 2, only Keri Russell, Rufus Sewell, and Allison Janney knew the wild developments in the last few scenes of the finale. The rest of the cast had received pages with redacted lines — which included that one huge twist.

“You need a genuine response to see if it’s going to do what you think it’s going to do,” says creator, showrunner, and executive producer Debora Cahn from set at Wrotham Park, in Hertfordshire, north of London. “And we got that genuine response.”

Indeed, when Ato Essandoh (who plays Stuart Hayford) heard the lines read aloud, his reaction was visceral. “I literally got up, threw the script down, and screamed!” Essandoh tells Tudum from his trailer between takes, while filming the penultimate scene. “I was like, ‘Oh, my God, Debora Cahn is a gangster.’ ”

If Season...