The inspiration for Slough House, the setting for Mick Herron’s award-winning, a-typical espionage series, may well be a real row of dingy-looking London shops but the disgraced spies toiling away in equally grimy offices above them is all thanks to Herron’s imagination. Which is funny, really, because, as Herron notes, “I don’t know why it caught my imagination, because it looks like somewhere imagination goes to die.” A dumping ground for MI5 agents who’ve made monumental mistakes on the job, Slough House is a conglomeration of warring personalities pushing paper and trying to stave off boredom.

Under the watchful eye of Jackson Lamb, one of the worst bosses in contemporary crime fiction, these downgraded agents putter around doing tedious paperwork, hoping for something, anything, that will send them back to the real business of running Britain’s Security Service. And yet, as Herron underscores, “It’s stated in the books over and over again, ‘you’re never going anywhere, you’re never going to get back to Regent’s Park,’ which is where they all want to be. And they still continue to think, ‘If I just do this one thing, maybe it will happen for me, not for anyone else.’ ”

The agents under Lamb’s command, these “Slow Horses,” are forever mired in the muck of their own failures. And it’s the minutiae of their lives that interest Herron more than the particulars of spycraft. “They could have been anybody, really,” says Herron of his office “full of rejects,” but “because I’m a thriller writer that narrows the choices. They could have been police officers but then I would have had to do actual research. And you’d have to get the procedure right because if you don’t, if you just make stuff up and you write about the police, you’re going to get into trouble. But if you make stuff up about the Secret Service, people say, ‘you seem to know an awful lot about that’ and then they assume you have some sort of experience.” Reading Herron’s series, from 2010’s Slow Horses through the January 2016 release Real Tigers during which the quasi-spies must band together when one of their own is kidnapped, one could be easily fooled into thinking that the author spent hours with real MI5 agents, recording their tics and trade secrets. But the one “field trip” Herron did take while writing Tigers, to a nuclear bunker similar to the one that plays a key role in the novel, didn’t result in any direct reportage. Instead, says Herron, “I visited it and then I just made everything up when it came to writing it because it didn’t actually fulfill the requirements of the plot.” In the end, he says, “reality didn’t work, so I just made it up.”

Whom Herron does turn to—when he’s not flipping through Christopher Andrew’s history of the Security Service, Defense of the Realm, which he calls “a fantastic book that reads like a novel”—are his espionage fiction forefathers, namely John le Carré and Robert Littell. “I won’t say that le Carré cast a shadow,” says Herron, rather “he cast a light over the spy genre that will continue for a long, long time.” For Herron, Littell is an author who “writes fantastic stories and there’s a lot of humor in what he writes, very cleverly plotted. I do like that, the humor, in a book.” All the Slough House books shimmer with Herron’s slick, black humor, the caustic wit of the characters like an oil sheen coating the plots. Both le Carré and Littell are “writing about people first and the situation afterward,” says Herron, “and that’s what matters. That’s what novels should be about: human behavior.”

The human behavior, and the humans who (mis)behave in particular, at the center of Tigers is especially fraught. Herron works with an ensemble cast, which he acknowledges has its advantages—“you can get rid of characters, bring new ones in, and there will be people who die”—and he compares the juggling act of balancing fictional egos to the artistic process of British filmmaker Mike Leigh. Herron mentions the quintessential British character actor Eddie Marsan, who was told by Leigh during the filming of High Hopes that he was the main character. “It was only during the premiere,” says Herron, who’d read an interview with Marsan, “that he realized that he was only a subsidiary character. Now, telling Eddie Marsan that he was the lead was a brilliant idea, obviously to get the best performance out of him, but also because it seems to me that that’s how we all feel anyway. We’re all the main characters in the narratives of our lives.” Herron’s MI5 flameouts like River Cartwright, Catherine Standish, and the odious IT guru Roderick Ho each believe they’re the hero. “Each one individually will crawl over the others’ backs to get back to their careers, back to where they were before, before they messed it all up,” Herron says. “I like the idea of everyone butting heads in that way.”

Since, as Herron says, it’s all about the characters and “the fact that they are failed spies is kind of incidental…,” it makes sense that the author holds a few of the MI5 misfits closer to his heart than others. Catherine, who takes center stage in Tigers, is a recovering alcoholic whose addiction landed her at Slough House and “in writing Catherine,” says Herron, “I wanted to provide the kind of happy outcome that life doesn’t necessarily give you.” There’s no point in trying to wheedle out the ultimate fates of the rest of the Slough House crew from Herron: “As far as I’m concerned, there are only two characters who are sacrosanct and I don’t think I’m going to tell you who they are, actually!”

Jordan Foster is a writer living in Portland, Oregon.