Last summer Jack Lowden met a fan. The star of Slow Horses was cycling past a pub at lunchtime when a man ran towards him. “He was so drunk that he carried his pint with him and dropped it,” Lowden says, laughing. “He stumbled and went, ‘Mate, I just wanna say I really love Sea Horses.’ ”

Nothing says that an actor is in a hit TV show more than being heckled by a drunk. Sea … sorry, Slow Horses, though, is not your usual smash. It is a classic “sleeper hit”. Hidden away on Apple TV+, it has taken three series for the spy drama to break cover.

“It’s bizarre that Season Three of something is what lights the touch paper,” Lowden says. “The reviews have been fantastic, but it’s always difficult when it’s not on ‘normal television’. Being on a streaming service gives a show a chance to grow.”

“Poor old River. He thinks he’s an action hero, but he doesn’t get the girl, he just gets shot.”

Now, quiet critical acclaim for Gary Oldman’s irascible Jackson Lamb — with his team of bumbling MI5 heroes — has exploded into global exaltation. Here’s the summary. Slow Horses, from the spy novels by Mick Herron, centers on Slough House, the home for MI5’s failures. If you’re working there, as Mick Jagger sings in the theme tune, you’re “finished, you’re foolish, you failed”.

Oldman’s performance as the grubby chief has been hailed as his career best. His merciless disregard for any social niceties is matched only by his scorn for his employees — and superiors.

Lowden plays one of those poor saps stuck with Lamb, River Cartwright. “He’s good, but he’s not as good as he thinks he is,” Lowden says, laughing. “Poor old River. He thinks he’s an action hero, but he doesn’t get the girl, he just gets shot.”

Oldman’s performance as the grubby chief has been hailed as his career best.

The rest of the cast — Saskia Reeves, Jonathan Pryce, Kristin Scott Thomas — are as classy as a night at the National, but it is the Oldman and Lowden double act that steals the show. “I’m 6ft 1in and Gary’s shorter,” Lowden says. “I’m horrendously aware of what we look like — before we even open our mouths, we’re next to each other in that sort of Rodney and Del Boy way. I find it very hard not to laugh.”

Lowden’s soft Scottish lilt may surprise viewers who have seen him play the Cockney gangster Kenneth Noye in the Brink’s-Mat drama The Gold and a young Tony Benn in A United Kingdom. Born in Essex, he grew up in Scotland and joined the Scottish Youth Theatre aged ten. His theater CV includes an Olivier Award for Ibsen’s Ghosts. His career puts him in the services frequently — as a soldier in the Belfast drama ’71 and in Christopher Nolan’s Dunkirk. He’s even one of the names tipped to play 007, something he waves away, insisting he’s still mourning Daniel Craig.

Lowden won an Olivier Award for his performance in Henrik Ibsen’s Ghosts, opposite Lesley Manville.

Should he inherit that mantle, his time as River will have helped. Early in his career Lowden’s good looks and tousled blond hair meant he was offered lots of posh young men roles, and Slow Horses has finally killed those offers. “I’ve always struggled playing earnest characters,” he insists. “I’m a terrible cynic. A lot of men my age are eternally disappointed that River isn’t wonderful at his job, but that’s what I love about playing him.”

The show celebrates failure, and if you hear an echo of that other savage take on British elite institutions, The Thick of It, you’re not wrong. Will Smith, the Slow Horses show-runner, worked under Armando Iannucci on the BBC spin doctor comedy and its American counterpart, Veep.

He is keen to stress how faithful he tries to stay to Herron’s books. The plots, the characters and a lot of the dialogue are rooted there but, unsurprisingly, with his former proximity to Malcolm Tucker, Smith makes it filthier. “I had to do a ‘f*** count’ and bring that sort of writing under control,” he confesses. “And there’s a fart line that Gary loves which has shifted from every season, but one day we’ll get it in.”

Slow Horses is not afraid to be funny, Lowden says. “I was obsessed with Only Fools and Horses when I was a kid and no matter how much those two brothers went through, the writer John Sullivan always whipped the carpet out from underneath their feet. River is constantly feeling that he can change his situation and get back to the sexier side of the track. But each series ends with him back where he started.”

“A lot of men my age are eternally disappointed that River isn’t wonderful at his job, but that’s what I love about playing him.”

This is a far cry from Oldman’s turn as a spy in Tinker Tailor Soldier Spy, yet also feels more real. Slow Horses is like spending the evening getting drunk with John le Carré and, at 2am, saying: “Come on, John, what was it really like being a spy?”

“That’s true,” Lowden says. The genius of the show, he adds, is that it appears to be a spy thriller but is in fact a workplace sitcom. The Horses get the dirty jobs and spend most of the time bickering. “The few interactions we had with ex-spies, it was really wonderful to know that our intelligence services really are full of people bitching about wanting a corner office rather than a cubicle or why don’t they get sexier assignments?”

Gary Oldman and Kristin Scott Thomas in the sleeper hit Slow Horses.

Intelligence advisers came and trained Lowden in spycraft. “My favorite thing that they revealed was their reliance on our industry for their recruitment,” he says, grinning. “TV and film make spies sexy and that’s why people join. I’d love to speak to them now that Slow Horses is out there, making it look grubby and boring.”

And now the whole world has latched on to this other side of spying — something that bemuses Lowden, considering it’s so very British. “It’s such a British brand of irreverent comedy developed over hundreds of years,” he says. “That’s why I really hope there is a season where someone comes in from the CIA. I would love Lamb to be sitting in the corner eating a doner kebab while some American is doing something fancy on a whiteboard to do with drones. That is perfect Slow Horses.”

Stephen Armstrong is a freelance journalist who writes for The Sunday Times, The Guardian, The New Statesman, GQ, and Esquire. His first book was The White Island: The Colourful History of the Original Fantasy Island, Ibiza