Gregory Piper, who plays gang member Ryan Pilkington, has returned to the police corruption drama for the first time since 2012.
His character, who was last shown in season one, featured as a bicycle-riding envoy between Tommy Hunter, played by Brian McCardie, and Tony Gates, played by Lennie James.
The six-part series, created by Jed Mercurio, explores the world of police corruption.
The 21-year-old, from Dudley, trained at Dramaworkshop at Chawn Hill Church, Stourbridge, where tutor Esther Stanford saw his potential.
Gregory said: "I was 13 at the time I landed the role in series one of Line of Duty. I went to a few auditions and then got a call while I was on holiday saying I had got the part of Ryan Pilkington.
"It was so surreal, and at 13 years old, you don't really take everything in and what is going on around you.
"Then I got this message out of the blue saying they wanted to bring back my character for season five. It was amazing news – I was in the middle of a rehearsal at college and I got this message from my agent saying 'you never guess what'.
"I started filming in September last year and finished mid-December. It was just great to meet all the new cast members on the series. Stephen Graham gave me advice on some of my scenes and everyone is just like a big family – they were all really welcoming.
"It was so top secret that I couldn't tell my friends and family so it was nice for them to all finally find out – everyone has been so excited, as am I. We are all sitting down together every Sunday to watch."
Esther, who is originally from Kidderminster, said: "Gregory came to me about three months before he got the Line of Duty role.
"He joined me at 13 and left at 18 and now he works as one of the assistant directors here. It's brilliant. He is a phenomenal actor.
"A man contacted me and asked if I knew of anyone suitable for the role and I put Gregory forward. He did three auditions and got the role.
"I knew he would be right for it – they initially wanted someone over 16 for chaperoning reasons.
"They got back in touch last year and he's now back in it."
The 40-year-old said she follows each episode – but focuses on Gregory, rather than the plot.
"It's difficult to watch because now I'm watching Gregory rather than following the storyline. I'm just delighted to see him back on the screen," she added.
"We've had a lot of success but Gregory is our biggest. He is one of the people who walked through my door and on the first day, I took his mother outside and said 'whatever it is, he's got it'.
"He's proved to be a talented actor and he is so modest. He's lovely and a team player – I'm sure he'll have a big career."
Gregory, a former Windsor High pupil, took to Twitter last month to reveal the news that he would be returning to the BBC drama.
The actor tweeted: "Balaclavas off, news is out, look who’s back and all grown up...it’s Ryan.
"So honoured to be asked back, what an experience to say the least. Roll on the rest of the series."