I thought I'd made a friend – then I found out she was a spy. Here's what I learned about real-world espionage

It was not Ava Glass's job to count the dead children. Her job was to hold ministers and intelligence services to account for the toxic cloud drifting from the burning factory towards the village, and the armed terrorists on the run. Happily, the children - the spies and soldiers now drinking tea in red tabards - were roles in a counterterrorism exercise. Earlier, their tabards had been yellow, labelled "Child"; now "Dead", they waited.

Cameras captured everything. The chiefs sat in the control room, observing the feeds. When there was no progress, the press warned of mounting panic. When civilians died, they lingered on the horror.

"Somebody would call me occasionally and say, 'This is'" – the placeholder brackets are audible – "'[Insert name of famous cabinet minister]', and sometimes they were, and sometimes they weren't," says Glass, who was working in counterterrorism communications at the time.

"Sometimes they were MI6 and pretending to be the cabinet minister. And they would say, 'We need you to write an article that says this and be as harsh as you like – we're trying to upset them. You're not being fierce enough in your coverage.' So then I would write something – often about my own boss – that was horrifically harsh. Because the whole idea was to put pressure on them. They had to be under pressure because in real life, the pressure would be unbearable."

Glass has many aliases. She wrote the young adult Night School series as CJ Daugherty, the Harper McClain crime thriller series as Christi Daugherty, and the Alias Emma spy series as Ava Glass. And now, she has written the thriller The Hiding Season as AC Glass. The last at her publisher's insistence: men, they claim, won't read a thriller by a woman. She's not convinced.

She's done well since leaving her role in counterterrorism communications. Her sales of spy fiction – "the most male genre I think I've ever encountered" – are strong, and the Alias Emma series has also been optioned for television. Producers are now circling The Hiding Season, which explores the damage that spies can do to those who become entangled with them and the cost of deception.

"All I ever wanted to do in my life was to be a writer. And do something good. And make a living. Those were my three career goals," Glass says.

Born in Texas, she paid her way through college writing before taking her first full-time position as a crime reporter in Savannah, Georgia. New Orleans was next. After several years "trudging down hot or very cold sidewalks and facing people down and trying to get information out of them", she came to England in "the dying days of that brilliant London media excess of the '90s" to work as an editor at Time Out. "I've never had more hangovers." But then the party ended.

She wanted to stay – London was now her home – but magazines and newspapers were shrinking. "And right then, a woman that I'd met only briefly and who I didn't even realise had my phone number called and said she'd taken a job working for the government. She was looking for somebody who could write about terrorism and not get scared. And she thought of me."

The first few weeks were spent settling in. One day, Glass bumped – literally – into a young woman in the canteen, who had also just started. A few days later, she ran into her in a favourite coffee shop. Shortly after, they found themselves on the same bus. They started hanging out.

"We didn't know anybody. And for two or three weeks, she was my office friend, and we chatted. She asked a lot of questions about my past, my background, how I ended up in England, just the kind of questions that people ask all the time, but lots of them. And I didn't really notice – until she completely disappeared."

Months later, Ava learnt this had been her final background check. "She was a spy. She was never my friend. It was never coincidental. And that's when I understood. This is who you're working with. They – we – will be anyone we have to be to get the information out of you."

She dedicated Alias Emma to her. "If I hadn't met her, I would never have written any of those books. She fascinated me. She was so young. I didn't know there were 25-year-old spies. I didn't know there were young female spies. We're not shown that. That was the beginning of my understanding that spies are nothing like we think."

Glass’s research into the spy genre uncovered the source of those misconceptions. "We don't expect female spies because art has never shown us female spies, or only very rarely. Books, particularly, have never shown us female spies."

Television and film do it better. Glass singles out Killing Eve and Alias as series with a strong female focus. Cinematic glamour, she argues, is not always inaccurate. Having experienced convincing deception, Glass finds a series like The Night Manager credible. "It's very glossy, and it takes place in high-end hotels, but if you are going undercover with the oligarchs, you have to fit in, and this is what the spies do."

Books, where they show female spies, tend to be about women accidentally finding themselves in that role. "They don't want to spy, but they have to. Every woman I met in that world was utterly focused. They didn't want to do anything else. This had been something that they wanted to do for years. People do not stumble into it. They fight to get into it."

The cost of that work fascinates Glass. "I knew spies in real life who had to lie to their families all the time. One I knew quite well was very devoted to his partner. And yet, his partner thought he worked at the Department for Transport. So he lied every day when he went home. I asked him, how can you? And it was one of the rare times when I felt like I got true answers from one of them because he just said, I hate it."

Secrecy constrains her. "There's loads that I can't tell people." Instead, Glass captures the emotion of real events in fictional ones. Despite her fondness for different aliases, she thinks being a writer would be a terrible job for a spy. A spy avoids drawing attention to themselves. Her aliases correspond in a way to the different lives she's led.

"I've written about a crime reporter in Savannah. She was 100 per cent not me and had none of my experiences, but all of my passion. All my love for that city goes into that story." Imagination fills the gaps. "I've never been a spy. But because I met them, I felt like I could then explore that."

When she sat down to write her first spy novel, she knew what she wanted: George Smiley "before all the bad stuff happened. When we meet Smiley in the le Carré novels, he's in his 50s, and he's bitter, and he's worn down. I wanted to write a George Smiley at 25, just starting out, a believer." And a woman, inspired by that first spy she met. Emma Makepeace is that character – the Emma of the series title. She gives us le Carré without the cynicism.

Glass now writes full-time, but she misses her spies. "Whenever I run into somebody from that world, which I have a couple of times now, they'll let me know. And they do it not by saying, 'I was a spy.' They'll say, 'Were you in the government when'" – she uses those audible placeholder brackets again – "'[Insert name here] – who's quite top secret – was there?' and the only way you would know them is if you were in that office at that time. And then, like, suddenly we know." She laughs. "And then we can have a real conversation."

Every conversation is potential material for a writer. The Hiding Season opens in a private mountain resort for billionaires, inspired by the Yellowstone Club in Montana, which her best friend told her about after working there as a caretaker. It lies empty outside two months of the year, "and I used to tell her it would be an amazing place to commit a murder because, I mean, no witnesses, no cameras. Miles and miles of land to bury the body. 'What if...?', which is, you know, the start of many a novel."

The protagonist, Maya Landry, must adopt a new identity after witnessing that imagined murder. Unlike Emma, she did stumble into deception. She has to hide the truth to save her life, and she hates it because it puts a distance between her and the people she comes to love.

Glass still has no idea if her first spy liked her or not. "But I liked her. And I was completely honest. I felt like such an idiot when I understood. What a fool. She must have thought I was so stupid that I didn't see it. And yet I have no idea what she thought or felt. If you had to do that day in and day out, would you eventually start to feel anything?"

The Hiding Season by Ava Glass is available in hardback and as an audiobook from 26 March.

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