Line of Duty returns for a fourth series of tormenting twists and turns in a new Sunday night slot on BBC One.

Vicky McClure and Martin Compston's Steve Arnott and Kate Fleming are back with a new thrilling investigation into police corruption, with fan favourite Adrian Dunbar overseeing all the drama as the head of AC-12.

Every series features a new antagonist for the team with Lennie James, Keeley Hawes and Daniel Mays all previously appearing in the show.

Series 4 has gone even bigger with Hollywood star Thandie Newton joining the team alongside Brit Jason Watkins.

Keep reading for a quick guide to the series four characters - in the cast's own words:

[Read more: Line of Duty series 4 episode 4 - All the burning questions]

[Read more: Vicky McClure and Jed Mercurio Line of Duty 4 interview - Catch up with the AC-12 team]

DCI Roz Huntley played by Thandie Newton

Thandie Newton in Line of Duty series 4 on BBC One

"I didn’t know anything about the role but I said I wanted to do it. The first thing I saw was series three. I don’t watch a lot of television - because I have kids the only thing I watch is SpongeBob or Paw Patrol! My agent said to me: 'Thandie, an offer has come in and if you ever want to work in British television, this is it'. I’ve been with my agent since I was 17 so when she said something like that, I sat up.

"So I watched it and was completely knocked out by it and Daniel Mays. I’m such a fan of him, as with Keeley Hawes and Lennie James, I’ve admired them for years. They had clearly been attracted to the material and after meeting Jed, I was in. He’s been a pilot, a doctor; he’s the most efficient human being I think I’ve ever met! He’s used all his skills and gone to the absolute pinnacle of each and I wanted to try and match that with my acting.

"I don’t have the opportunity to work in Britain very much because there are a lot of period dramas and they often don’t have calls for women of colour so I went to Hollywood."

[Read more: Catch up the series 3 finale of Line of Duty]

Tim Ifield played by Jason Watkins

Line of Duty series 4 star Jason Watkins

"I did a bit of research. I got to know a policeman up in Cumbria via Twitter and he said if you ever need to do any research then come and see us. I was filming something nearby so I nipped up the coast and I met a Forensic Coordinator.

"Tim’s a forensics expert, not only is he a very detailed and brilliant practitioner but he is meticulous and professional. Often these guys can solve the case themselves and I think because they have the ability to do that, sometimes there’s friction between other departments."

Steve Arnott played by Martin Compston

Line of Duty season 4 Martin Compston

"A bit of time has passed since the end of series three, we’re all back at the office, and Steve’s back in his waistcoat being his arrogant little self again. He’s really pushing for a promotion and things are just a wee bit frosty between him and Kate. They always have that love/hate relationship, when they really need each other they are there for each other but they really can annoy each other at times. They were probably my favourite scenes, when Vicky and I are getting on each other’s nerves."

Kate Fleming played by Vicky McClure

Line of Duty series 4 Vicky McClure

"She’s been promoted to the same rank as Steve Arnott and she’s pretty raring to go actually. She’s got a lot to prove and she wants to make sure she makes Hastings proud and we see her ambition and drive are as strong as ever. Her personal life is not ideal but is settled.

"Kate and Steve have a continued strain on their relationship but they really look after each other. It’s a bit like a brother and sister relationship."

Ted Hastings played by Adrian Dunbar

Adrian Dunbar on Line of Duty

"It is getting a little deeper this time. We’re not seeing any of his personal life but we are seeing his professional life put under a huge amount of pressure. He’s trying to balance out the fact that he’s very loyal to the concept of policing and law and order in general and he believes in it but his problem always is that people who are lower or even higher than him are miscreants."

DC Jodie Taylor played by Claudia Jessie

Claudia Jessie in Line of Duty

You may recognise Jodie from daytime drama WPC 56 or minor roles in Call the Midwife and Jonathan Strange and Mr Norrell. Jed Mercuiro very rarely lets characters pass by without reason, so we suspect we'll be seeing more of the young officer in Roz Huntley's team as the series progresses.

Michael Farmer played by Scott Reid

Scott Reid in Line of Duty

Best known to TV fans as Methadone Mick in the comedy Still Game, Scott Reid switches to drama for the role of Michael Farmer. Has Michael been the victim of a stitch-up? Or is he playing an evil game?

Nick Huntley played by Lee Ingleby

Lee Ingleby in Line of Duty

A popular BBC drama star, well known for his roles in The A Word and Inspector George Gently, Ingleby wouldn't have signed up to the show for a minor role. But how will Roz Huntley's husband fit into the mystery and does he have any secrets of his own?

Neil Twyler played by Mark Stobbart

Mark Stobbart in Line of Duty

Another rising star in the cast, you may have seen Mark in Vera, Doctors, Skins, Utopia or the film Almost Married.

ACC Derek Hilton played by Paul Higgins

Paul Higgins in Line of Duty

The most senior officer in the new series and a returning character from series 1, but how much do we trust ACC Derek Hilton. In the words of Ted Hastings, how do we know when he's lying - when his lips are moving! 

Detective Constable Jamie Desford played by Royce Pierreson

Royce Pierreson in Line of Duty

Royce starred in the 2016 series of Our Girl alongside Michelle Keegan and also made an impact in the Bafta-winning BBC Three drama Murdered by My Boyfriend.

He will play a newcomer in the AC-12 team in episode 3 - but we'll have to wait a while to find out whether he's a good un or a wrong un.

WPC Maneet Bindra played by Maya Sondhi

Maya Sondhi in Line of Duty

The fan favourite played a key role in bring down dastardly Dot Cottan in series 3 and she's already popped up several times in series 4. Played by Maya Sondhi, you may recognise her from Citizen Khan, DCI Banks, Silemt Witness or Family Affairs.

DCI Ian Buckells played by Nigel Boyle

Nigel Boyle as Ian Buckells

Friend or foe? We're still in the dark about the new head of Operation Trapdoor and where he stands on AC-12. He previously appeared in series 1 opposite Tony Gates (Lenny James), but does he bear a grudge against Steve, Kate and Tes after previous events?

Jimmy Lakewell played by Patrick Baladi

Peter Baladi as Jimmy Lakewell

Nick Huntley's lawyer on speed dial, smarmy Jimmy Lakewell, is being played by Bodies and The Office star Patrick Baladi. Why was Nick so keen to get Jimmy involved, even before Steve Arnott's attack? Could Jimmy have been balaclava man or being behind the attacks? We definitely don't trust him and reckon he's involved in this plot much deeper than hr first appears.

[Read more: Line of Duty series 4 - All You Need To Know]

[Read more: Line of Duty - Why did Hollywood star Thandie Newton sign for the British TV show]

[Read more: Thandie Newton reveals Line of Duty changed her opinion on the police]

Photo Credit: BBC Pictures