The Line of Duty star tells us all about the show's new villain and what we can expect in series four of the hit crime drama...

Line of Duty returns to our screens on Sunday for a highly anticipated fourth series and anti-corruption unit AC-12 has a new crooked copper in its sights!

After proving a ratings hit on BBC2, the crime drama will be coming to BBC1 this year and the cast reckon this is the best series yet.

Thandie Newton (Westworld) will star as DCI Roz Huntley, the anti-corruption unit’s latest target, and Martin Compston, who plays AC-12’s DS Steve Arnott, says she’s just as intimidating as her character!

‘I don’t know if she’d appreciate me saying this but Thandie’s kind of like a hard Mary Poppins,’ said Martin. ‘She’s really really sweet but you wouldn’t mess with her! She’s an absolute darling, she came on and she was full of enthusiasm, everybody took to her right away and that’s great because it gives everybody else a lift.’

Martin also goes on to reveal that DCI Roz Huntley could be AC-12’S toughest adversary yet…

‘Jed Mercurio writes really strong female characters,’ says Martin. ‘Roz Huntley is smart and such a well-crafted character. She’s a really tough cookie!’

Superintendent Ted Hastings (Adrian Dunbar) DS Steve Arnott (Martin Compston) and DS Kate Fleming (Vicky McClure) are back as AC-12 take on a fresh case.

Vicky McClure and Adrian Dunbar will be returning as AC-12’s DS Kate Fleming and Superintendent Ted Hastings, while Jason Watkins and Lee Ingleby also be joining the cast for the new series…

Now good friends after working together on three previous series, Martin, Vicky and Adrian usually read the scripts together when they first receive them, yet this year was slightly different.

‘I was filming something else at the time,’ explains Martin. ‘Usually I’d be champing at the bit to read them but the problem is once you start reading them the scripts are so good, you can’t stop!

‘I told Vicky I was going to have to leave it until got the job I was on done, but she was desperate to talk to someone about it. We were both filming in Glasgow at the same time so she was meeting me at night going “Just read it!” because again there are some big big twists!’

In a spectacular climax to series three, corrupt AC-12 officer Matthew ‘Dot’ Cottan was revealed as The Caddy and died in a hail of bullets, in a storyline that had run since series two.

‘This is definitely is a whole new case,’ said Martin. ‘But with Jed’s writing nothing is ever quite how it seems when it starts to unfold. There’s a rumour going around that The Caddy isn’t actually dead, but trust me, he is!’

‘I heard that was another rumour that Dot would kill Steve at the end of the last series. I pulled Jed up on it and he said “No no”. I don’t know if he started the rumour so I would keep my mouth shut, but he assured me it was never really a possibility.’

Yet even if his character was going to be killed off, Martin says he wouldn’t tell us.

‘One of my friends fell out with me because I accidentally told him a plot line,’ he added. ‘He was gutted,  people really enjoy being left with the suspense week to week.’

Line of Duty returns on Sunday 26 March on BBC1