Des star David Tenant isolated himself from his co stars in the ITV drama in a method acting move to ensure their icy distance felt real.
Daniel Mays, who plays DCI Peter Jay on the show, spoke to Phillip Schofield and Holly Willoughby on This Morning on Monday.
He said that David, who plays the chilling serial killer David Nilsen, deliberately stayed away from the actors who played police officers, rather than bonding with the rest of the group on set.
Daniel said David's performance was "one of the most eerie and captivating performances he had ever worked across from."
And the former Doctor Who star even learned how to mimic Dennis' real signature, so deeply immersed was he into the role.
"He kept separate to all the actors playing the police, we had bonded as a group as you can imagine all of the ensemble playing the police, and that comes across in the police as well," said Daniel.
"But certainly when David stepped out, when we were doing the interrogation scene in the first episode, we instinctively all knew he was keeping himself separate and focusing on what he has to do
"And a group of actors we completely respected that."
He went on: "To sit across from David Tenant in that role, I've been acting for 20 years and it's one of the most captivating and eerie performances I've ever worked across from.
"For my money, he is giving the performance of his career, he is absolutely stellar in this performance.
"He even perfected his signature, there was one scene, I don't think it made it into the film, he had to sign the interrogation at the end of the statement.
"He nudged me and he had his phone in his hand and said, 'What do you think of that Dan?'
"It was absolutely identical to Nilsen's signature, just to give you a bit of insight into how detailed David is in his work."
* This Morning airs weekdays on ITV at 10am