...Is based on a real life event but didn't exactly happen like they show in the series. "On one occasion, when Douglas' colleague
Robert Ressler was in a cell alone with Kemper, Kemper—who at the time weighed 300 pounds (136 kg)—noticed the apprehension in Ressler after he had pressed a hidden button repeatedly to call a guard to open the cell yet not received a response and told him "Relax. They're changing the shift," but remarked: "If I went apeshit in here, you'd be in a lot of trouble, wouldn't you? I could screw your head off and place it on the table to greet the guard."
[71] Ressler verbally sparred with Kemper until the guard arrived, ultimately leaving unharmed with Kemper displaying no physical aggression. Kemper also stated afterwards that he was joking, but Ressler never entered a cell alone again with Kemper and it became FBI policy to interview serial killers in pairs"