Since David Fincher's cerebral crime procedural Mindhunter quietly arrived on Netflix last Friday, it's gathered all the critical acclaim and online buzz you'd expect, and a few Tumblr memes you might not. Chronicling the early days of the FBI's Behavioral Science Unit, where the term "serial killer" originated, Mindhunter follows agents Holden Ford (Jonathan Groff) and Bill Tench (Holt McCallany) as they conduct a decades-long criminal science study, interviewing serial killers in the hopes of producing meaningful research about the psychology of murder.

If you binge-watched all ten episodes of the show in a single sitting last weekend, you were probably left with a lot of questions after that unsettling finale and vaguely horrified by the realization that Season Two isn't coming until 2019. Fortunately, I also had the chance to speak to Mindhunter's co-lead McCallany this week about reuniting with Fincher, developing the show's central relationship with Groff, and what killers we might expect to see in Season Two.

McCallany initially assumed that Bill Tench was a supporting role, based on the parts he had previously played for Fincher.

At first, I was just sent a couple of scenes between Bill and Holden, from Laray Mayfield who casts all of David Fincher's projects. I was in David's first film Alien 3, all the way back in 1990, and then I did Fight Club, but both very peripheral parts. Fight Club has become this iconic film and I think it's a masterpiece, and we knew even when we were shooing it that we were a part of something very special. But my character is very much a supporting character, so when I first got the material for Mindhunter, I didn't really understand how important a character Bill Tench was in this story. But because it was David, I wanted to [audition] anyway, so I shot a tape and sent it to him, and at that point he sent back the bible of the show, which Joe Penhall (the writer of Mindhunter's first season) had written, an overview of the entire series arc, and at that point I understood that this guy is one of two protagonists. I was thrilled that David was inviting me back in that kind of a part, because it represented a big promotion for me! It was almost like I graduated.

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Penhall has written a five-season plan for the show, and there's no reason to think Netflix won't follow through on that promise.

The five-season bible I read was approximately a 60 to 70 page document that just laid out the broad strokes: a much more detailed rundown of Season One, and then an overview of what potential subsequent seasons would look like. Hopefully we'll have five seasons, if all goes well, and if you're gonna work with somebody for five years, you'd better make sure it's somebody you have a good experience with. It can be arduous at times—Season One of Mindhunter was a ten-month shoot, which is long, and for much of that time we shot six-day weeks. I was really lucky to be with a director that I like and admire, and a co-star, Jonathan Groff, who is one of the nicest guys in show business. He's just such a sweetheart, everybody loves him, and he's very professional and very prepared and very respectful to David and to the crew. He's just a pleasure to be around. I kind of lucked out on this one.

McCallany, Groff and Fincher were all conscious of the need to steer Holden and Bill's relationship away from buddy cop cliche.

We've seen this sort of older cop/younger cop dynamic in a lot of Hollywood films—even in one of David's films, Seven, with Morgan Freeman and Brad Pitt; Training Day with Denzel and Ethan Hawke; Colors with Robert Duvall and Sean Penn… The trick is to not try and borrow anything from those movies; you have to discover what is interesting about this specific relationship. Jonathan and myself are very different as people, we have different personalities, different backgrounds, and that contrast gave us a lot of things to play.

It felt like the scenes with the serial killers were so intense, and so dark, that Jonathan and I made a concerted effort to try to find moments of humor in our relationship, little moments of levity to juxtapose with the intensity, and David really embraced that idea. It really grew out of a natural rapport: I'm not saying we're the Laurel and Hardy of FBI agents, but when you have two guys that are just of different generations, we have different personalities, there's a lot to mine there.

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Though McCallany and Groff's characters are inspired by real men—Robert Ressler and John E Douglas respectively—Mindhunter is distinctly not a biography.

It is loosely inspired, because the writers wanted to give themselves leeway when it came to the agents' personal lives. We are dealing with real killers, so they wanted to be very authentic with their stories and their crimes; if we're doing Charles Manson or Ed Kemper or Richard Speck, volumes have been written about these guys, so the writers make an effort to get those details are precise as possible. But when it comes to the characters' personal worlds: Bill's in a failing marriage, he has an adopted son who has psychological problems and he has difficulty communicating with, which creates problems in the marriage, and none of that is taken from Mr Ressler's life. We don't want to take liberties with the crimes that were committed by these guys; it wouldn't be fair to the audience, and frankly it wouldn't be fair to the victims of those crimes. But when it comes to the personal lives of the agents, and is Holden gonna break up with his girlfriend, it's a different equation.

Sadly, Mr. Ressler died in 2013; it would have very much interested me to spend time with him, but I read all five of his books. I spent time at Quantico in Virginia at the FBI headquarters, and met some of the guys there who were in the Behavioral Science Unit, and some who knew Ressler. I also got to spend a weekend with John Douglas at his house in Virginia; he's a very prolific guy with a tremendous amount of experience in this area, and of course he knew Robert very well, so that was really valuable to me.

Bill is so willing to go to bat for Holden because before Holden came along, he was so disillusioned by the FBI.

My take on Bill from the start is he's a guy who is floundering. He's forgotten why it's important that he be an FBI agent; he still enjoys law enforcement, but his passion for it is not what it once was. He's just been beaten down by the bureaucracy of the FBI, because he's not interested in the internal politics at Quantico, and—if you'll allow me to put it this way—the brown-nosing that you have to do to move up through the ranks. Bill's not interested in all that.

When Holden enters his life this young guy's energy, and his enthusiasm, his passion for the work and his intelligence and his good ideas eventually make Bill kind of stop and reassess. He realizes he's become set in his ways, and after the meeting with Kemper, he realizes that this kid is really onto something. From that moment onwards, there's a real change in Bill, especially when he gets the validation from Anna Torv's character of how important this work is. At that point, it just cements in Bill's mind that they have to find a way to get authorization, even if it means sticking his neck out for Holden. His presence in Bill's life is a major turning point.

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It was immediately clear on set that Cameron Britton's performance as Ed Kemper was something special.

It was incredible to watch, and we understood that this is this young man's breakout performance. He wasn't known to a wide audience before, and his characterization of Kemper was so spot on; I mean, he not only looks like him, but he talks like him, he sounds like him, he has the mannerisms, it it was just an incredible piece of casting, and then Cameron really took the ball and hit it out of the park. Because Ed Kemper is the first killer that was interviewed by Ressler and Douglas, and the first killer that we interview on our show, it was so important that we get a strong performance, and boy did we! But I think we did very well with all the killers, we got terrific performances across the board.

Season Two is already in pre-production, despite the lack of an official renewal announcement from Netflix.

It's been very strange, the season two of it all, because I was told we were doing one, but then I was told Netflix hadn't made an official announcement, so if I'm asked about it I should say I'm not sure! But… I am sure, because I know they're already locking down locations in Pittsburgh, and doing all the things they have to do in terms of pre-production.

I know that David's working on the scripts, and I do have a sense of it, but I think I'll probably get myself into trouble if I talk too much about what Bill's journey will be. Some stuff involving my family, some stuff involving my son, and [Holden and Bill] will of course continue to do the study, continue to meet new serial killers, which may include people like [Son of Sam killer] David Berkowitz… But I think if I say too much more, I might get a call!

From: Esquire US