Line of Duty star Craig Parkinson dropped a major hint that Kate Fleming, played by Vicky McClure, is hiding a huge secret in an interview on This Morning in 2020.
Speaking to hosts Eamonn Holmes and Ruth Langsford in August last year, Craig, 45, revealed you can "never trust Vicky McClure" hinting Kate could even be H.
"If you haven't seen the show, it's going to be really exciting for you and it's going to be really exciting even if you have seen it, to piece together little clues and little twists here and there," he said.
"And the main thing is, don't ask anybody about it, don't listen to anybody about it and above all, don't trust any of these characters - Especially Vicky McClure!"
Craig, who hosts the BBC's Obsessed with... Line of Duty podcast, has already watched the hotly-anticipated series finale which could be the last ever episode of the popular police drama.
But he refused to reveal the identity of the mysterious crime boss H, who is thought to be unmasked at last in the programme.
On his BBC Radio 2 show, Dermot O’Leary asked Craig, 45, if viewers were "in for a belter".
"I think it's going to shock people," replied Craig.
"I think people are going to be frustrated, it’s exhilarating, it’s exciting… these aren’t soundbites, I’ve seen it four times, honestly, and it really stands up to repeat viewing."
Craig added that the episode would "break hearts" with fans already in meltdown with their many predictions and theories as to who H is, and what will become of AC-12.
Speaking on Sky News' Backstage Podcast, he revealed: "I'm so proud of the guys for going back after the break, getting it done without any illness and delivering one of the iconic great British dramas that we've produced in the past few years.
"Talk about timing, we all need AC-12 in our life right now, albeit I'm sure they're going to break some hearts at some point."
Meanwhile, Martin Compston, who plays DS Steve Arnott, has dropped a major hint about the identity of H in a brand new video posted to his Twitter account.
Martin, 36, can be seen in the video singing the alphabet, but when he gets to the letter H, Mark Bonnar, who plays DCC Mike Dryden, pops up beside him and stares into the camera.
"Oh, hello Mark," says Martin, before the pair both burst out laughing.
* Line of Duty airs Sunday night on BBC One at 9pm
