To mark Stephen King’s 70th birthday, the BFI are running a season of King material on screen – including previews of Mike Flanagan’s Gerald’s Game, released on Netflix next week, and the first two episodes of Mr. Mercedes, the Audience Network series starring Brendan Gleeson and Harry Treadaway. Paul Simpson buckled up…

We’ll be reviewing the episodes of Mr. Mercedes elsewhere on the site, but suffice it to say that they hold up incredibly well on the big screen, from the horror of the opening scene through the intimate moments between Brendan Gleeson’s retired cop Bill Hodges and his next door neighbour, Ida (beautifully played by Holland Taylor) to the escalating tension of the battle of wits between Hodges and Harry Treadaway’s Brady Hartsfield. It’s not for the faint of heart – no punches are pulled in the opening massacre, and the incestuous overtones of Brady’s relationship with his mother are front and centre – but it exerts one hell of a grip… and the audience at the BFI would, I suspect, have gladly watched the third or more episodes.

Harry Treadaway was the invited guest for the Q&A (which wasn’t a formal “on the record” press conference so we’ll keep this report focused purely on the TV series, rather than the general discussion of terrorism that ensued) and fielded questions confidently about his preparation for the role, and the sometimes quite terrifying things he learned about killers in our society today. He could tell instantly that he would find director Jack Bender interesting to work with, following an initial Skype conversation, and after filming an audition tape with his brother and girlfriend (using one of the controversial scenes from the pilot referred to above), he was cast as Brady.

For him the biggest challenge was “trying to empathize with someone who has no empathy”, and he researched the role extensively before filming began (although it sounds as if there’s a bookseller in Hampstead who was rather disconcerted by his reading list!). When he arrived for filming in South Carolina, the papers were filled with the case of Dylann Roof, who had killed nine people in a church in Charleston. “That for me was the scariest thing – people committing violent acts for no reason just because they feel like it… that suburban horror, the feeling that it’s the next door neighbour that can do it.” Treadaway admitted “it was a weird and disturbing headspace to be in.” To stay “on an even simmer in the evenings” after filming, he watched three TV shows every night: UFC Fighting, Shark Tank (the American equivalent of Dragon’s Den) and Fox News!

King’s novel was “an incredible blueprint – my copy is tattered and littered with stickers and scrawlings – and it was such a wonderful thing to be doing a story where you could dip in and read a chapter that incorporated the scene you were about to tell and go and do it, because it was so full of his inner thoughts.” A lot of the physical attributes of Brady came from the book. “We haven’t done anything that’s not in there – it talks about his dark eyes and he’s an insomniac, and he’s incredibly wired the whole time.”

He praised the “amazing cast and crew” but had to point out that he didn’t have that many scenes with Brendan Gleeson – “we’re existing in different worlds the whole series pretty much. I would be turning up on set and Brendan would be leaving.” The first time they were on set together “was the weirdest thing – I’d spent five months trying to hide from him!”

King himself came to set during the filming. “He loved the early footage and wanted to come on board and support it,” Treadaway recalled. “Which was obviously amazing for our production. It was great.

“Someone who’s written 80 of the darkest novels ever written, I didn’t know what to expect. I was thinking, am I going to meet some horrible man? But of course he was completely lovely and charming… and tall! He kept telling me I was killing it, which was marvellous, and I was in awe of someone who could write that much.”

The evening concluded with Treadaway leading the entire audience singing Happy Birthday to Stephen King.