WHEN you need answers, go straight to the top. In this case Superintendent Ted Hastings – played by the brilliant Adrian Dunbar – who heads up the AC-12 anti-corruption unit in BBC drama Line of Duty.

As the fourth series drew to a close in 2017, there was a gasp-inducing moment that saw many viewers emit a visceral howl of anguish. There was Ted looking a bit shifty, right when it seemed that everything has been tied up neatly in a bow.

Whether Ted could be “H”, the corrupt police officer with a hand in almost every diabolical conspiracy running through Jed Mercurio’s award-winning police procedural since it began in 2012, has become the subject of feverish speculation among the show’s fans.

Adding fuel to the flames, a BBC promo shot shows Hastings handcuffed to fellow AC-12 officers DS Steve Arnott and DI Kate Fleming, played by Martin Compston and Vicky McClure, with a tagline that reads: “Crime needs an inside man”.

As Dunbar’s dulcet Northern Irish tones drift down the line, I opt to tackle the swirling rumours head-on. Please say it’s not so, Adrian …

“At the end of the last series there was this indication that Ted was under some kind of suspicion and that is followed through on,” says Dunbar. “It does not look very good for Ted, I have to say.

“Everybody is going to have to hang tough with the whole thing because it unravels bit by bit across the full series. He does come under suspicion and that is followed through on.”

Gulp … The fifth series of Line of Duty, which gets under way this evening, will centre on organised crime groups (that’s “OCG” in Line of Duty acronym-loving parlance).

It’s been two years since we last caught up with AC-12 – Mercurio has been off making hit BBC thriller Bodyguard starring Richard Madden – and as the new series begins viewers are immediately thrust into the action when a balaclava-clad gang ambushes a police convoy.

Fans will be delighted to learn there are plenty of classic-style Line of Duty moments to look forward to over the coming weeks, including jaw-dropping twists, shock betrayals, grisly crime scenes, marathon interrogations – and, yes, those delicious Ted-isms.

Dunbar’s character has garnered quite the following for his phraseology and quick one-liners (“Hastings – like the battle!”) with the actor deftly introducing a clutch of Northern Irish colloquialisms such as “fella” and “now we’re suckin’ diesel” that didn’t appear in the original scripts.

“I had no idea these things were going to catch on,” he laughs. “Or that people would be playing pub games with them.” Ted Hastings bingo? “Yes, it is a bit mad. I have got a good one coming up – I think it is in episode one – but I’m not going to tell you what it is. It is a real bang-on Belfast one.”

The eldest of seven children, Dunbar, 60, grew up in Enniskillen where his late father was a carpenter and his mother worked as a cook. As a teenager he aspired be a musician but then found acting after a cousin introduced him to a local amateur drama group.

Dunbar studied at Guildhall School of Music and Drama in London and went on to roles in critically-acclaimed films such as Hear My Song and The Crying Game. He’s been a regular on our screens over the past four decades, popping up in A Touch of Frost, Silent Witness and Ashes to Ashes.

Since 2012, he’s played Hastings in Line of Duty and his fondness for the character is palpable. Showrunner Mercurio is said to have initially envisaged the AC-12 boss as more of a shambolic figurehead in the beguiling manner of TV detective Columbo, as played by Peter Falk.

When Dunbar was cast and brought his own take on Hastings, Mercurio saw the strength in having a proactive and dynamic leader who would also be a mentor to Arnott and Fleming.

“He [Mercurio] realised the series needed that person, somebody who is bright, tough, sharp and could handle difficult scenarios,” says Dunbar.

“Ted is old school. I think people like that about him. But he is also quite vulnerable and insecure sometimes, so it is not all kick the door in. He is tough where the letter of the law is concerned but he is not entirely a tough person himself.”

Dunbar says he drew inspiration from the style of great Scottish footballing managers such as Jock Stein, Bill Shankly and Alex Ferguson when breathing life into Hastings.

“He has a good man management style and sense of morality,” he says. “That kind of character who has a way with people and they trust him. They know that ultimately, even if it is to his own detriment, he will always do the right thing.”

Line of Duty is set in an unspecified, generic Midlands town. The first series was shot in Birmingham with subsequent episodes filmed in Northern Ireland, chiefly around Belfast.

That suits Dunbar down to a tee. “I love going back to Northern Ireland,” he enthuses. “Me and Martin [Compston] often talk about the idea that we might get a shift over to Glasgow some year, but that has not happened as yet. We could have a right laugh if we got to Glasgow as well.

“Belfast is great. It is a small city and we can get around it easily enough. We have got to know it really well over the years. Of course, for me, it’s great because I get to see friends and my family can come to see me – all that kind of craic.”

The actor has often spoken about striving to keep his own accent for roles whenever possible because he believes it lends greater authenticity.

“It is always important to get regional accents up there,” asserts Dunbar. “When you think of the likes of Bill Paterson or the guys who came out of 7:84 bringing Scottish accents to the screen and not feeling the necessity to change their accents.

“But also, I think it really fits the part [in Line of Duty]. I wouldn’t try and do it if it didn’t. It fits the part to have [Hastings] speak like that.

“There are aspects about it that are useful such as the fact he was an RUC officer and he would have had training in anti-terrorism which figures a lot in AC-12.

“When he comes to head up AC-12 he is an outsider. There is nobody in the rest of the police force that you would think would be able to get to him on a personal level or who would know him from before. All those things help to locate him as being from Northern Ireland.”

Compston revealed during an earlier series that his estuary English twang as Arnott began to slip slightly, seeing his own Greenock accent begin to creep in, due to exhaustion as the gruelling filming schedules took their toll.

At least Dunbar doesn’t need to worry about that? “No, that’s right,” he chuckles. Although that’s not to say Line of Duty’s famed knife-edge interrogation scenes are a doddle to shoot. In fact, far from it as Dunbar can testify.

“Those things are very tough,” he says. “I have to learn sometimes 25 pages at a time. The takes can last 20 minutes – we do big, long takes. You always hope that you get a couple of days in between so you can learn the next one because you can’t keep everything in your head at the one time.

“Police are allowed to look at their notes so, some of the actors, they can look down at their notes and see what is coming up next. But I can’t do that because I don’t wear glasses in the part, so if I look down, I can’t see anything. I have to learn everything. Which is fine but it is tough.”

Line of Duty has earned a reputation for its high calibre of guest stars who have included Keeley Hawes, Daniel Mays and Thandie Newton among others.

The latest series is no exception with Boardwalk Empire and This Is England actor Stephen Graham joining the cast as the elusive “Balaclava Man” and Rochenda Sandall, known for her roles in Black Mirror and Love, Lies and Records, as AC-12’s formidable suspect, Lisa McQueen.

Does bringing in names like this help keep things fresh? “I think so but the main thing is that Jed keeps the standard of his work up,” says Dunbar. “Unbelievably, he seems to be able to do that every time. When we come in and read the new scripts, we are always very excited.

“But then, of course, you have to marry those new characters with great actors. Martin and Vicky are two grounded and truthful actors. That standard has been kept up all the way through.

“The believability of the series is very important to us. The likes of Stephen and Rochenda are bang on the money as regards the type of actor and acting we love.”

You might imagine Dunbar would be inundated with fans hoping to glean spoilers but that’s not the case. “Never,” he insists. “People come up and say: ‘We absolutely love Line of Duty – don’t tell me anything.’ They like to watch it in real time and want to be thrilled by the next twist or turn.”

As for what he enjoys most? “The acting I get to do with the role. The decisions I have to make and all the things emotionally that he has to go through. Actors are like race horses. They like to run – they don’t like to be hanging about. The part gives me lots of hard work to do and that’s enjoyable.”

Line of Duty begins on BBC One, tonight, 9pm