Series six of the BBC One crime drama comes to an end on Sunday, but a number of characters remain under suspicion in the hunt for the "the fourth man" - an individual commanding a network of corrupt officers behind the murder of journalist, Gail Vella.
Acting Detective Superintendent Joanne Davidson, played by Scottish actress Kelly Macdonald, is one of those suspects.
The Trainspotting star, 45, told the PA news agency even she did not know about a pivotal twist about her character's parentage until halfway through filming.
She said: "Jed Mercurio man, he is troublesome. He keeps a lot to himself. I feel like Martin (Compston, co-star) always heard about things first. I felt like he would get the lay of the land.
"I don't know if that was just because he is so on top of everything and understands everything acutely and what the connotations are.
"I didn't have the full story on Jo. I just knew that things hadn't been great for her. Jed kept saying, 'She is this enigmatic figure' and he was being enigmatic himself.
"By the time we came back after the first lockdown I knew what was up."
Speaking about which stars know the identity of H, she added: "The main cast know. I don't know about everybody. H knows who H is... Like I said, I don't know the whole cast, but I know the top cast."
Despite a number of revelations about Davidson linking her to criminal groups, Macdonald said she believed her to be a good person.
"I think she has got a good core. I really do," she said.
"She has just been brought up in tricky circumstances and hasn't really had the chance to be the person that I think she knows is in there. She has had to put a lid on so many things.
"It is a pretty solitary life. We know about the relationship with Farida but clearly that was an unhealthy one and pretty one-sided.
"If you are so buttoned up you can't have a real relationship with anybody else."
Macdonald admitted she had become more "reclusive" after her debut at the start of the sixth series, and had not watched any more since.
She added: "It has just been odd. I can't tell you how odd it has all been. I just got told the numbers from (last Sunday) and it is a lot.
"It's just a lot of people watching it and I am still in the living room, not going online and not watching. It feels like a slightly different dimension."
The Line of Duty series final airs on BBC One and iPlayer on Sunday at 9pm.