Hugo Weaving has told me too much about Slow Horses. He said he wouldn’t – “Ask whatever you like. Of course, there are things I can’t possibly tell you” – but then he did. And now, like the journalist in season one of the British spy thriller, I’m in danger of being tracked down by the Dogs, MI5’s internal security service, if I discuss the plot and what comes next.

However, it’s not Weaving’s problem. He’s thrilled to have stepped into season four of the Apple TV+ series, a rare foray into big budget international TV for the acclaimed Australian actor.

“It’s a superior piece of television,” he says. “The show is about family, not just this season, but the whole thing. Slough House is the dysfunctional family that is trying to get back in the good books of the institution.

“And that’s why this show is interesting because all of our heroes are failures in some way, and some of them don’t know why they’re there. They’re all in purgatory. It’s a purgatorial world that exists for human beings who f---ed up in this system. We all know what that’s like, where our weaknesses keep coming back to bite us and burn us, and we don’t even know what they are. That’s why it’s a great series.”

For the uninitiated, Slow Horses is based on Mick Herron’s Slough House series, which follows the “rejects” of the British security service MI5 who have been banished to the crumbling Slough House (hence their nickname “slow horses”). As their boss, Jackson Lamb – an impeccably dishevelled Gary Oldman – likes to tell them: “Working with you has been the lowest point in a disappointing career.”

Despite that, they are quicker on their feet than the MI5 agents they regularly tussle with, outwitting MI5 deputy director-general and head of operations Diana Taverner (a deliciously brittle Kristin Scott Thomas) and outrunning the Dogs, the service’s internal security division.

Ever since the show premiered in 2022, it’s been a critical hit, loved for its thick streak of black humour and, like Lamb, unsentimental approach to its characters. It has been commissioned for a fifth season and also heads into the Emmy Awards with nine nominations under its belt – including outstanding drama series, lead actor in drama for Oldman and supporting acting for Jack Lowden, who plays young agent River Cartwright.

Which means Weaving enters the series with impeccable timing. This season is adapted from Spook Street, the fourth book in the series, with his character Frank Harkness the American leader of a mercenary group. He has links to assassinations around the world, but this time he has the Cartwright family in his sights: not only River, but his grandfather David (Jonathan Pryce), a retired MI5 intelligence official.

“I loved the character of Frank,” says Weaving. “I thought he was great. He’s like your mysterious uncle that you might have heard quite a lot about, or nothing much about, but you’ve got a sense that other members of the family are deeply disturbed by. He’s the American uncle who’s been in the army and when he appears, he’s got a certain charm and a certain relaxed nature, but actually, he’s profoundly disturbing.”

Weaving doesn’t appear until the second episode – you’ll recognise his low growl of a voice before you see his face – and even then, it’s not until the final couple of episodes that you get a full picture of who this mysterious man actually is.

“I would not define Frank as a villain,” says Weaving. “Yes, I understand that’s the role he plays, but it’s interesting. In Mick Herron’s books, he’s CIA, he’s an agent ... who works in the field. He’s gone renegade – the book is quite different – but he’s gone renegade for a reason. So in a way, he’s very smart, he understands how constricted these agencies are.

“The whole Slow Horses thing really is about a dysfunctional family fighting against an institution [MI5] that is trying to tell them what to do. Frank has a similar thing. He’s a dysfunctional man who feels like he needs to work outside the boundaries of the CIA.”

Weaving’s connection to Slow Horses is also tied to family. He is second cousins with the show’s creator Will Smith and Weaving has a photo of them together when Smith was about five and Weaving was about 13. For his part, Smith has described Weaving as his “awesome famous relative” and said when he read Spook Street, Weaving was the first person he thought of to play the villain.

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Weaving, despite being flattered – “I’m hoping he thought I might be able to do the role” – disagrees with this description of Frank.

“I don’t see him as a villain,” he says. “And I can’t see him as a villain anyway because I have to play him, and he has to be a human being. Otherwise, it’s just stupid. I know they were at every point trying to push him into being the villain, and he is, but I can never define him like that. Otherwise, he loses his humanity, and he needs to be human because of the interesting links to the Cartwright family.

“He’s got history with the Cartwrights. He’s got history with MI5 and that’s what’s interesting about his injection into the season. He’s a significant addition to that family.”

And speaking of significant additions, I’m keen to see if Weaving can solve one of the great TV mysteries for me: in the opening credits of shows, who decides who gets to be a “with” and an “and”? In Slow Horses, Weaving is an “and” wedged between “and Jonathan Pryce” and “with Kristin Scott Thomas”. Which is better: “with” or “and”?

“I don’t know,” he ponders, before deciding that “with” is indeed something special.

Maybe he knows more, but like any good spy, he won’t reveal his sources. Either way, I’m outta here, before the Dogs catch up with me.

Season four of Slow Horses is now streaming on Apple TV+.

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