"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.