Line of Duty guest stars: ‘No one is safe – Jed can kill characters left, right and centre’

The role of the Line of Duty guest star is one of the most coveted in British television. Each series, actors who join the drama know it will be watched by millions on Sunday nights, that it will be packed with shoot-outs and quick-fire dialogue, and that they’ll be added to a roll-call of impressive alumni, from Thandie Newton to Stephen Graham. There’s just one catch – more often than not, they get brutally killed off.

It’s a formula that has gripped audiences since the first series began in 2012, drawing in 4 million viewers from the start. It broke records for BBC Two, got bumped up to BBC One, and now, entering its sixth season, it’s expected to top its previous record of 13.7 million viewers. The show, created by Jed Mercurio and set in the fictional police anti-corruption unit AC-12, is as famous for its rolling guest stars as it is for its labyrinthine plot and inscrutable acronyms. “From the initial pitch, the series concept was that the investigators would be the returning characters and the guest lead under investigation would appear in one season only,” says Mercurio. “It gave us the creative freedom to take the guest lead’s arc to a point of no return.”

The first ever guest star to go on the beat with series regulars Vicky McClure, Martin Compston and Adrian Dunbar was Lennie James. He played Tony Gates, a hotshot officer involved in a deadly hit-and-run who was being probed by AC-12. The investigation came to a gruesome end – with Gates walking in front of a truck. “We were the first ones to go to the moon,” says James, who is currently in Texas filming The Walking Dead and recovering from the crippling winter storm that hit the state. “We filmed in Birmingham, and from there on they shot in Belfast. So for a number of reasons the first one was unique.”

The show’s success was completely unexpected. “We had no idea that we were doing anything other than a really well-written cop show by Jed,” says James. “My overriding feeling, at the time, was what a fantastic time I had with Vicky, Martin, Aidy, Neil [Morrissey] and Craig [Parkinson]. We hung out with each other, we lived close to each other, we ate together, we worked together, we had a ball and we’re friends to this day.”

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James says Mercurio had to fight for the show. “It wasn’t a given,” he says. “BBC Two, at the time, thought they were taking a risk and their expectations weren’t particularly high. But then the networks were fighting over themselves to put it on.”

Line of Duty was hardly enshrouded in secrecy then – but now, the plot is as top secret as a Marvel movie. The embargoes are set in steel, with screeners for the show accompanied by a long list of spoilers that cannot be revealed. And sure enough, our writer Fiona Sturges was asked to unhear a few things that the new guest star, Kelly Macdonald, let slip in her recent interview. Mercurio insists it’s “no different from any other production in only releasing official information when it’s confirmed and keeping scripts confidential”, but Daniel Mays – who played firearms officer Danny Waldron in series three – tells me there was a “huge amount of pressure to keep things under wraps”.

“It was apparent from the off that I had to keep it under my hat,” he says from East Finchley, where he’s been homeschooling his children, an experience he compares to “World War Three”. He recalls a close shave where he nearly revealed that Keeley Hawes’s iconic series-two character, Lindsay Denton, was making a comeback. “When I started,” he says, “Vicky and Keeley were both Bafta-nominated for series two. And literally one of the first ever tweets that I sent out was like, ‘Wow, you can’t move for Bafta-nominated actresses on this gig. Congratulations, Vicky and Keeley.’ I came into work the next day and there was a knock on my trailer. The producer came in and said, ‘Listen, about your Twitter feed… people are not meant to know that Keeley is back on the show.’ I wanted the whole ground to open up and swallow me. I thought I’d let the cat out the bag. I deleted the tweet. Luckily, it didn’t get any traction at all. But I was seriously considering deleting my Twitter account entirely.”

Mays was polished off in just one episode, so he had to do a “weird acting exercise” and “lie to journalists” to make out that he would be starring throughout the series. “It was a very odd thing to go through,” he says. “It was such an absolute cliffhanger at the end of that episode. I remember reading that script for the first time and my first question was, ‘Oh my God, does he make it?’ And then Jed said to me, ‘I’m really sorry, Danny. This is the remit. This is what we’re asking you to do.’ It was an audacious thing to do, because I was effectively announced as the new lead and yet they were gonna kill me off. That’s the great thing about the show – no one is safe. Jed can kill characters left, right and centre, you know, and he’s not scared to do that.” Mercurio adds that “Danny was aware up front of his character’s arc. We all loved working with him so it turned out to be a poignant moment when he completed his last scene – our feeling he was gone too soon was mirrored by the audience, which was the intention.”

Did Mays tell his family? “Yeah, I told my wife,” he says. “I’m terrible at that. I was in Rogue One, the Star Wars film, and the director honestly said to me, ‘You are not allowed to tell anyone you’re in it.’ I went, ‘No, of course.’ The first person I told was my then 12-year-old son. You have to be very careful about who you tell and you have to trust those people.”

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Anna Maxwell Martin, who joined as Patricia Carmichael, a senior anti-corruption detective, for the last few episodes of series 5, struggled to keep things hush-hush, too. “I kept forgetting about the secrecy thing, to be honest,” she says, “I had a few panics a few times. I’m very much a non-filtered ‘talk first, think later’ sort of person.”

Martin says she was “over the blimmin moon” when she got the part. “I didn’t even read it, just said yes. I would have made tea in the background for all I cared. It turned out I had a bit of the olde interviewing to do [the show’s lengthy interrogation scenes are iconic]. Again, I loved it, I had a real giggle, everyone in the glass box was lovely, we had a great director in Sue Tully, so I just enjoyed it. It’s rare in acting that you actually get to really blither on, so I relished it. It’s such good fun to play an absolute cow, or very misunderstood person.”

She admits, however, that she only ever read her own scenes, so didn’t know what was happening in the wider storyline. You can hardly blame her. The plot is so intricate that one actor turned down the guest lead role because of the “mental” level of detail, as Macdonald recently put it in an interview with The Guardian. It got to a point, she told this newspaper, where she couldn’t imagine doing anything other than learning lines. (“Now I look back and think” – she does a double air-punch – “‘I did it!’”)

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James says the complexity of the plot has become a character in itself. “It wasn’t quite as bad when we were doing it first time around,” he says, “but I watch it now, and particularly Compston, he gets episodes where all he’s doing is talking in acronyms.”

“I think Jed sometimes just does it to torture you,” Compston told This Morning. “It kind of becomes a wee badge of honour, trying to learn them and stuff.” Many glossaries for the show’s police jargon have appeared online over the years.

Line of Duty was a turning point in the careers of many of its guest stars. Every time the first series is repeated on BBC One, James is approached by a new set of fans. “I remember very clearly going to the new Tottenham Hotspur stadium when it was rebuilt to see one of the early games,” he says, “and everybody was Line of Duty up to the eyeballs.”

Mays, who was Bafta-nominated for his one episode, is hugely appreciative of what the show has done for his visibility. “I definitely recognised it was, to that point, the best part I’d ever played. It was one of those opportunities that comes along every once in a while that you have to grab with both hands. The fact that I got a Bafta nomination for it was just incredible. I’m proud of that. It was an absolute game changer for me. It raised my profile commercially but also, in terms of an acting performance, it really showed what I could offer. The roles have come in thick and fast ever since.” He laughs. “I have played quite a few coppers now.”

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The increased attention hasn’t always been pleasant, though. “I found the paparazzi thing that goes with the show a shock and quite upsetting,” says Martin. “I’ve never had a publicist or PR and never courted all that so I didn’t like it. It took me by surprise, I ended up in a chase through north London because I thought ‘You can sod off.’ Eventually we stopped, and he got out and said, ‘God, you drive like a nutter!’ I taught my kids to flick them the Vs.”

Mays is still a huge fan of the series. “I’m still proud every time it comes on BBC One,” he says, “and that it’s gone on to become this absolute juggernaut.” His advice to future guest stars is to “relish every second of it”. “It is writing of the highest level,” he says. “I hope it goes on and on and on because I don’t think it’s outstayed its welcome yet at all.”

James has also watched “every single minute of it”. “I was really gutted that I wasn’t coming back, even though I knew I wasn’t because I got hit by a truck,” he says. “I was genuinely put out not to be amongst their number anymore because we had such a good time. So I watched it to see what they were all up to. The show has never stood still. It’s challenged itself and moved forward. When Keeley came in, she took it to another level and what they did with Danny, killing him off as quickly as they could, was just brilliant. I haven’t been surprised by television like that in a long time. And then they bring in Thandie and then Stevie G. I mean, they just keep upping and upping and upping their game.”

And what about Patricia Carmichael, one of the few guest characters who wasn’t killed off? Will she be back? “No, Pat won’t be back,” says Martin. “She’s having a lie down.”