WARNING: Contains spoilers
Dudley actor Gregory Piper plays gang member-turned student police officer Ryan Pilkington in the police corruption drama.
After murdering undercover cop John Corbett, played by Stephen Graham, as part of the organised crime group in the last series, Ryan is back with an even bigger role in the drama.
Now he is working within the force under Joanna Davidson, played by Kelly Macdonald, whose team is investigating the murder of journalist Gail Vella.
At the same time anti-corruption unit AC-12 begins investigating Joanna and Operation Lighthouse as they suspect a cover up.
Gregory, now 23, trained at Dramaworkshop at Chawn Hill Church, Stourbridge, where tutor Esther Stanford saw his potential.
After the most recent episode aired he was praised online by the show's creator Jed Mercurio, who is from Cannock.
Gregory, who is currently living in Halesowen, said:"You never know what's in the mind of Jed, it's very exciting and I've had such a lovely response from everyone about it.
"When I was watching the second episode my phone was going off so much I had to put it away so I could focus on the episode, it's been amazing."
Back in season one his character was the envoy between Tommy Hunter, played by Brian McCardie, and Tony Gates, played by Lennie James.
He tried to cut off the fingers of DS Steve Arnott, played by Martin Compston, who was disrupting the operations of the Organised Crime Group. DS Arnott has since remembered this after looking into Ryan's past.
The show which aired last Sunday saw Ryan's tampering with the investigation reach a new level as he swerved a police car carrying a suspect into a reservoir, before drowning colleague PC Lisa Patel.
Ryan tried to also drown the suspect Terry Boyle, played by Tommy Jessop, but sharp-eyed DI Kate Fleming, played by Vicky McClure, followed the car and found it in the river just in time to make Ryan pretend he was actually saving Terry.
Jed Mercurio tweeted this week saying: "Great performance from Gregory Piper who was in and out of a freezing reservoir dozens of times over three bitterly cold nights, always with total dedication to the work and never a single word of complaint. Thanks from all on Line of Duty."
Gregory said the water scene had to be filmed in November as the production was paused during the first lockdown.
He added: "We filmed the water scene in the middle of November so the water was very very cold.
"Knowing that the scene would be shot I had turned the heat in the shower right down to try and acclimatise. It was really really exciting.
"The crews were absolutely lovely, they had hot water bottles and blankets ready for when I came out."
Gregory also spoke of his shock at the production being stopped last March and how he has been living with his girlfriend and her family due to the pandemic.
"Because of lockdown I've been living with my girlfriend and her family in Halesowen so after each episode airs we have a call with my family," he added.
"We started filming in March 2020 in Belfast and it got stopped then resumed in September until the middle of November.
"That was a shock as I didn't think something would stop the production of it."
And each week the family gets together to watch the show before Gregory calls his own family to discuss it after.
He added: "It's very exciting to know it's coming, I saw little bits before but it's brilliant to be able to watch the whole thing now. My heart feels like it's going to beat out of my chest every week, it's very surreal.
"It's a reminder of everything I've done."
In a preview of tomorrow's episode viewers saw DI Fleming question Ryan about the unusual route the police vehicle took to get the suspect home, and why his window was the only one open when the car was pulled from the water.
The episode will also see Davidson continue buckling under pressure from AC-12's investigation.
So far Ryan has managed to cover his tracks, but as Gregory said when asked about the remainder of the series, "anything could happen".