Mother of God! It's as shocking a twist as any AC-12 have yet encountered, but Kelly Macdonald - the latest guest star to join the Line of Duty cast -  had never seen the show before she was offered the part.

The Trainspotting and Boardwalk Empire star, who joins the police thriller as the guest lead DCI Joanne Davidson in season 6, dropped the bombshell just days before the drama makes its eagerly anticipated return to BBC One. 

“I spent a number of years in New York filming and I missed out on a lot of TV and British pop culture. There were people on TV I didn’t recognise and Line of Duty was one of those things that happened while I was away,” she admitted.

The AC-12 team in season 6 of Line of Duty BBC

Macdonald was talking at the BBC’s online launch event for the eagerly-anticipated new episodes, which start on Sunday, March 21.

Rather than releasing the whole series at once, viewers will have to tune in the old-fashioned way, with a new episode airing weekly. Which is probably just as well because we’re not sure our blood pressure could cope with a full-on Line of Duty binge.

Macdonald was joined by her co-stars Adrian Dunbar and Vicky McClure and the show’s masterful writer Jed Mercurio. Here is what else we learned from the event…

Covid completely changed filming

“When I saw the trailer the other day I was quite shocked… There was quite a bit that had slipped my mind.”

Adrian Dunbar, who plays AC-12 boss and fan favourite Ted Hastings, admits that the hectic schedule changes forced by Covid restrictions made an extra challenge when shooting the latest series as they frequently skipped between episodes to speed up production.

“Jed had to often remind me exactly where we were because we were jumping between scenes and episodes,” said Dunbar. “It’s difficult to know how to pitch something when you’re moving between episodes like that. It was strange.”

McClure, who plays Hastings' colleague Kate Fleming, detailed the extensive efforts that were put in place to keep the cast and crew safe during filming.

“There was a traffic light system as you went on set. We built a set with ventilation. There was even talk of us doing our own make-up. I’m sad that didn’t happen for one day, just to see how that panned out,” she laughs.

“There were a lot of changes and the schedule was the biggest change for the actors. We were shooting with different directors and different episodes on the same day."

Adrian Dunbar in Line of Duty season 6 as Ted Hastings

Despite these extra and unusual obstacles, the show’s creator and writer Jed Mercurio refused to complain about the added stress of filming in 2020, arguing that they should “count our blessings” they got to do their job.

“It’s just great timing for us that we’ve managed to get through the production. There’s a lot going on in the country at the moment, there’s a lot of people out there suffering enormous hardship,” said Mercurio.

“If this is a distraction from that, we’ll be honoured.”

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Nobody is safe in season 6

Vicky McClure and Kelly Macdonald in Line of Duty BBC

Gates, Denton, Cottan, Corbett. We’ve bid farewell to some big characters over the last decade, but the trio at the heart of the series – Hastings, Fleming and Arnott – have so far survived unscathed. Just.

But there are no guarantees in Line of Duty and Mercurio warned that he has toyed with the possibility of bumping off one of the holy trinity.

“It’s never far from my thoughts,” teased the writer.

“We all get on brilliantly, but everyone knows we are all serving something bigger than ourselves which is Line of Duty. Something that everyone knows about the show is that nobody is safe. That is what keeps everyone on the edge of their seats.

“I know it would be a sad day, but all the main cast know that it’s possible. We’re mates. We laugh about it, we joke about it. It’s something that no one would relish but everyone would understand.”

Vicky McClure is a TikTok mischief

“I’ve got mixed feelings about this stuff that she gets up to,” said Dunbar.

The Irish actor is talking about Vicky McClure’s cheeky habit of taking pictures and videos on the set, including a now legendary dance sequence in AC-12 HQ.

“She just ambushes you,” said Dunbar. “She asks you now again, ‘I’m doing this thing for charity, just say this thing’ and you do it because you trust her. Then on that one occasion, she said that she would play this bit of music and told us to dance along.

“I thought what is this all about… TikTok. TikTok. Within a couple of days a million people had watched it. You have to be careful with Vicky. Take the rough with the smooth!”

Martin Compston’s lockdown waistline

Martin Compston as Steve Arnott in Line of Duty BBC

Steve Arnott’s waistcoats have become so popular they deserve their own spinoff series, but for actor Martin Compston, there was an issue with his costumes when he returned from lockdown.

“When we started the job at the beginning of this year, I was in pretty good nick! Then lockdown happened - we all ate too much, drank too much,” said the Scottish star.

“When I came back to the UK and had to quarantine, I asked our lovely costume designer to leave Steve’s suits in my room so I could try them on as they were all tailored. That was a really tough day - I was bursting out of everything!”

McClure teased: “He went back to [his home in] Vegas in lockdown. I asked him the other day about being out there and he asked me not talk about it. But he obviously had a few [drinks], as we all did and chilled out for a few weeks.

“When he came back he had to get an exercise bike in his flat and ate soup for two weeks.”

Is Kelly Macdonald the new ‘H’?

Kelly Macdonald on the set of Line of Duty BBC

She may never have watched Line of Duty before filming, but Kelly Macdonald was soon brought up to speed by her friends and family.

“People have been really excited from the get-go and screaming in my face, ‘Are you H? Are you H?’ and I didn’t know what it meant. I thought I should get something with H to wear,” she said.

“Everybody asks and is desperate to know, but before you can say anything they add, ‘I don’t want to know, I don’t want to know’. You just have to let them get on with it.”

And what did MacDonald bring to the Line of Duty family? Surprisingly, the answer is jigsaws.

“Kelly brought in a jigsaw that will stress me out forever,” said McClure.

“It was the hardest jigsaw I’ve ever seen. Sometimes you sit around for half an hour waiting and there would be a piece driving me nuts for ages and then they’d call me onto set.

“We started it before lockdown. We were doing them way before everyone started buying them. We were so rock and roll.”

Season 6 will delve into the show’s past

Martin Compston as Steve Arnott in Line of Duty BBC

Fans of the show who love spotting old characters on the suspects wall or references to classic moments in previous series are in luck, because Mercurio promises that AC-12 will be delving deep into the archives in season 6.

“When we want to make a connection with the past, often the thing we don’t quite know straight away is the exact detail,” he said.

“So we then have to go into the old scripts for the exact locations and dates when something happened.

“Even better if we have got some visual reference, if there’s some way we can show an image from the AC-12 files of the past and that’s something we do in this season. We delve into past cases and like any police unit they keep records so when they dig into them, we see reminders of previous seasons.”

The big ‘H’ reveal

It was confirmed in the final episode of season 5 that there were four figures using the mysterious codename ‘H’. We’ve already discovered three: Hilton, Biggeloe, Dot. But one ‘H’ is still out there.

Will we get a definitive answer this year?

“I don’t think there’s anything that can be said about that. What you get from the trailer is that this is still a ghost haunting AC-12,” said Mercurio.

“But it haunts the characters in different ways. If you chase something that has tormented you there are some people that will redouble their efforts and some that want to drift away from that endeavour and who want to give up.”

Watch Line of Duty season 1-5 on BBC iPlayer.

Line of Duty season 6 launches on Sunday, March 21, 9pm on BBC One.

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