Tom Cullen on Why 'Knightfall' Is the 'Best Thing' He's Ever Done (Exclusive)
Larry Horricks/HISTORY

The Knightfall crew moved heaven and earth to bring the show to screen... and so did its star, Tom Cullen. 

The show was picked up for 10 episodes straight-to-series in January 2016, signifying History's confidence in the Jeremy Renner-executive produced project, despite a massive set fire halfway through filming, and a Welsh actor who auditioned for the role of a warrior monk looking "like a mix between a gerbil and a Christmas pudding." 

"Honestly, I looked like a turkey!" Cullen boldly confesses to ET after a night out promoting the show in Los Angeles. "I had just come out of Christmas, and I just looked like Christmas, do you know what I mean? I was the human equivalent of Christmas, so I had to do a lot of work in order to make myself look like a knight." 

The actor was in the gym five hours a day for three months "to make myself look less [like a] turkey" before filming began in June 2016. But in addition to the dynamic characters and complex relationships featured on the show, there's battle scenes galore, meaning Cullen didn't just have to look the part, he had to be able to act it as well. 

So, just as Landry and his fellow knights began their search for the Holy Grail, Cullen and much of the cast -- Sam Hazeldine (Godfrey), Simon Merrells (Tancrede), Padraic Delaney (Gawain), Ed Stoppard (King Philip IV), Olivia Ross (Queen Joan), Julian Ovenden (William De Nogaret), Sabrina Bartlett (Princess Isabella), Bobby Schofield (Parsifal) and Jim Carter (Pope Boniface VIII) -- were enrolled in a three-week "Knight School" program, as Cullen likes to call it. 

Larry Horricks/HISTORY

"We'd do boxing and circuit training in the morning, then we'd go horse riding, then we'd come back and do some more fighting and then we would go to the gym," he recalls. "That's where we all met and bonded. If you want to become really good friends with people very quickly, do five hours of boxing and circuit training, and you'll be in pain together. That's the way to do it." 

Cullen also went through mental preparation, "bathing" himself in literature about the Knights Templar and the political and economic cultures of the Middle Ages, before traveling to France to complete his research. The 32-year-old set off on a self-guided "Templar Secret Tour of Paris" -- though his work trip wasn't strictly business. 

"My mate was living there, and I hadn't seen him for ages, so we just went out and got really drunk," he says, unapologetically. "So I woke up in this horrendously hungover state, and I'm like, 'Right, where do I need to get to in Paris to start this Templar tour?' And I got my Google maps out, and I was like, 'That can't be right. What the hell?' I found out that I was sleeping on the site of the old temple!" 

"I was actually sleeping where these men slept, which was kind of the whole point of me being there. I just wanted to feel it in some way," he explains, seemingly pausing to replay the significance of the event in his mind. "The fact that I was right there and I had no idea was bizarre. That's nuts." 

Larry Horricks/HISTORY

Cullen's dedication to the project came to a crashing halt in August 2016, when a fire broke out on the backlot of the Czech Republic's Barrandov Studios, destroying a reported $4.5 million in Knightfall sets. 

"It took 400 men seven days a week for four months to build those sets, and they cried watching it burn," Cullen remembers. "The sets were enormous. It was the biggest set in Europe. They built Medieval Paris, and the idea that all that amazing work and craftmanship was going to be gone, was kind of unthinkable. The first thing that goes through your head is just like, 'How do we continue? How do we carry on?'"

According to the actor, however, the crew "didn't drop a day" -- and became stronger in the process. 

"The next day, they cleared it, and just built it all again. Watching those guys, having put their heart and soul into these incredible sets, it was really amazing. And to watch them go back at it, and rebuild those sets, it was inspiring to all of us," he shares. "It just brought us together and made us a very tight family, all the way from the actors, right down to the people who were out there in the freezing Czech winter building, so that we could film there." 

Production wrapped nearly a year ago, in December 2016, completing 10 episodes of what Cullen calls "the best thing I've ever been involved with in my life." 

Larry Horricks/HISTORY

"This was the first show I did where when we started, we had two episodes, so it became a very collaborative environment in which the writers are feeding off of what we're doing, we're feeding off what the writers are doing, and they're feeding off what the sets look like and the costumes, and the world that we're creating," explains Cullen, who noted that Downton Abbey and Black Mirror were already successful shows when he joined the cast. "You create what your show is."

And for those looking to fill the Game of Thrones void with Knightfall, Cullen -- who stars alongside Kit Harington on HBO's mini-series, Gunpowder -- would advise viewers not to draw comparisons. 

"I've known Kit since I was 18 and we went to drama school together. We're great friends, and I'm a massive fan of him, and I'm a huge fan of Game of Thrones, [but] Knightfall is Knightfall. We worked very hard to try and create our own vibe, our own story, our own feel to the piece," he insists.  

"The battle scenes, all the way to the really amazing, delicate relationships that play out in the show, [form] a really rich world full of conflict and great politics," says Cullen, whose girlfriend, Orphan Black star Tatiana Maslany, is already a big fan of the show. "If you last through the 10 episodes, it's going to be a real high-octane journey. I hope that people enjoy it as much as we enjoyed making it." 

Knightfall premieres Wednesday, Dec. 6, at 10 p.m. ET/PT on History.