Mindhunter stars Jonathan Groff and Holt McCallany reveal what it's like to work with David Fincher

“It’s Fincher”, he says. “So it’s like, ‘yes, of course’, I would sign up for that for ten years.”

The former Glee star had auditioned for the role of Napster founder Sean Parker in Fincher’s Facebook biopic The Social Network, but was turned down as he didn’t have the sinister edge that the director was looking for (the role was eventually filled by Justin Timberlake).

But this time around he was deemed as the right fit for intellectually curious protagonist Holden Ford, an FBI agent in the Behavioural Sciences Unit, who tours the US in the 1970s interviewing the most notorious serial killers in modern American history. It took him a while to adjust to a more dramatic role.

“I smile a lot in my real life,” Groff says. “I think [Fincher] knew that was one of the things he was going to have to drum out of me was my innate desire to be likeable.”

Ford is an earnest, young agent - inspired by real-life FBI man John E Douglas - driven by the idea that understanding criminal psychology will help the bureau preempt crimes and solve cases.

But there is still a significant amount of resistance to Ford's ideas, and Bill Tench (Holt McCallany), his gruff, weathered superior, has been worn down over the years in his battle to convince skeptical colleagues that psychology is an avenue worthy of exploration.

“Holt and I are so different,” Groff says of his co-star. “We’re completely opposite personalities, but we really connected and had this kind of innate humour in our relationship.”

Jonathan Groff & Holt McCallany at the Mindhunter Premiere in London

McCallany, a bear of a man with deeply expressive blue eyes, worked with David Fincher many years ago, taking on smaller roles in Alien 3 and Fight Club, but getting the call for Mindhunter represented a seismic shift in his career trajectory.

“This was especially gratifying, because it was almost like getting a promotion - and especially to have it come from David,” McCallany says. He shares Groff’s admiration for their director: “I think that he would be successful in any walk of life - if he had been a coach of an NFL team he would have three Superbowl rings”.

In the past, Fincher has been criticised for his notoriously demanding approach to film-making, but Groff is quick to defend his director’s reasoning.

“Every time you know he’s perfecting it, and you know why he’s doing it, whether it’s an acting thing or a writing thing that we’re tweaking, he just wants it to be as good as it can be on that day.”

Director David Fincher

Fincher serves as an executive producer on the series, along with actress Charlize Theron, and he directs four out of the 10 episodes - but his fingerprints are all over the entire series. Despite handing the reins over to different directors for six of the episodes, he was ever-present on set.

“The actors, the the crew, and even the directors felt like we were in Fincher school. We had three very different directors, very talented directors in their own right - but we were all sort of fulfilling, or trying to fulfil, David’s vision,” says Groff.

“David was there the whole time that we were in Pittsburgh [where the series was filmed], and would often come in the mornings even when he wasn’t directing and help set up the first shot and talk through the day.”

“I think Se7en is a masterpiece,” says McCallany, “and I like Zodiac [his 2007 Jake Gyllenhaal-led thriller]. I think our show kind of lives in some world that borrows from both of those - and it was very important to David that we tell the story in as authentic a way as possible.”

This meant getting the serial killers right. The show is set up around a series of interviews that Douglas' conducted, as documented in his book Mindhunter: Inside the FBI's Elite Serial Crime Unit, and the writers were keen to ensure the depictions were as accurate as possible.

“They’re not the kind of stylised hollywood serial killers that are tremendously witty and charming, because most of them are not like that. Most of them are these deeply fractured and tormented, deeply depraved, sad and sadistic men," McCallany says. "The [scenes with serial killers] are written based on trial transcripts, interviews and actual quotes, so they’re meticulously reconstructed to represent these guys as they really were; the crimes as they were really committed."

It is this attention to detail that makes the series so deeply chilling; in the first couple of episodes, viewers are introduced to real-life serial killer Ed Kemper, who is currently serving a life sentence in California after murdering a number of young girls, his grandparents and his mother before handing himself in to police in 1973. He is not at all what you would expect; oddly thoughtful and warm. You might forget he was capable of such heinous crimes if he wasn't so forthright in discussing them.

"Ed Kemper was a really intelligent and affable kind of a guy, but they’re not all like that. We're going to meet some different ones that have very different personalities," McCallany said.

Groff adds that his empathy-first approach to acting helped him to his character's drive to understand these reprehensible killers.

"With every character the first thing I want to feel is empathy. Even if it's someone that's doing something horrible, [my approach is to find] a way in emotionally so that you can understand who they are, and why they’re doing what they’re doing."

Mindhunter is available on Netflix UK. Watch now