Vigil star Paterson Joseph, who plays Commander Neil Newsome, has admitted that the cast were kept in the dark about the murderer's identity during filming.
On The One Show today (September 21), Joseph spoke to presenters Alex Scott and Jermaine Jenas about making the new hit BBC One series, without knowing the full story.
"We didn't know! Most of the cast didn't know who the murderer was. In fact, the person who is responsible lied to us," he revealed.
"We've got to bring that person back into the fold, because we thought they were one thing and they were actually another."
Vigil follows the crew of a nuclear-powered submarine where a death occurs on board shortly after the mysterious disappearance of a fishing trawler. DCI Amy Silva (Suranne Jones) must investigate the death and find out whether it was accidental, or there was something deeper at play.
While he was trying to avoid giving any spoilers ahead of Sunday's final episode, Joseph did say this might not be one for fans to binge-watch.
"I wouldn't advise it," he warned, calling the show "claustrophobic". He added: "I'm tense watching it and I know pretty much the ending."
The series was created by the same team behind Line of Duty, another BBC crime drama that has been met with widespread praise. Joseph called Vigil "the wet Line of Duty".
He also addressed what he thinks is behind the runaway success of the new series, which has already become the BBC's most watched new drama of 2021.
"The strange thing about it of course is that it's a sort of lockdown drama," he explained. "We've got this show that's about people in a confined space. On top of that, there's this awful crime and possibly a murderer amongst them.
"What's brilliant about it is that a lot of people resonate with that feeling of being trapped. And if danger was to be part of that, it just ups the stakes."
Vigil airs on BBC One, with the finale airing Sunday, September 26 at 9pm.
Watch: Who are Extinction Rebellion and what do they want?
You Might Also Like