They visit with Berkowitz to try to gain insight on the BTK Killer (a self-chosen moniker that the killer came up with himself, standing for 'Bind, Torture, Kill), finding out about the compulsion to return to the scene of a crime, and also getting him to reveal that his public play to be perceived as insane was a facade.
You may not initially recognize Oliver Cooper, the actor who plays Berkowitzhis face is covered in prosthetics to make him look closer to the real guybut you may have seen him in some other projects. Most prominent of those projects, in fact, is a movie called Project X, which was released in 2012. A found-footage, Cloverfield style take on a giant teen party movie, Project X was bashed by critics, but has become something of a favorite among a certain subset who were the target audience (mostly late teens) at the time that the movie was released.
Cooper's role in that movie was as Costa, a sweater vest-clad obnoxious clown who builds the party up, leading it to beshocker!way bigger than ever intended, approaching dangerous levels.
Cooper also played Wheeler, the smarter-than-you-think freewheeling pot dealer in the wildly-underrated Amazon Prime series Red Oaks. The show is undoubtably a much more endearing role than Project X. Red Oaks is a take on the '80s comedy, with touches of John Hughes movies, Fast Times At Ridgemont High, and more.
As you've probably gleamed from the above, most of Cooper's experience has come in the comedy world. In addition to Red Oaks and Project X, Cooper's also had smaller roles in movies like Office Christmas Party and The Hangover Part III, as well as a recurring role alongside David Duchovny in Californication.
His background primarily in comedy makes the turn as a heinous serial killer that much more notable and impressive. Channeling into that kind of a mindset, attempting to capture that sort of energy and mania, even just for a single scene, can take a serious mental toll.
