I first met my detectives in much the same way we meet people in real life – I got a glimpse of them from a distance, with a few basic impressions that told me very little about their true personalities.

When I sat down to write the first book in the Cooper & Fry series, Black Dog, there were only three things I knew about Derbyshire police detectives Ben Cooper and Diane Fry, who were to become my central characters.

Rather than add another world-weary, middle-aged detective inspector to the world of crime fiction, I’d decided to make my characters young and junior. Ben and Diane are in their twenties at the start of the series, and they’re both detective constables, on the bottom rung of the ladder in CID.

Because the Peak District locations are fundamental to the books, I wanted two contrasting pairs of eyes to explore the setting. So I made Ben Cooper the local boy who grew up in the area and knows everyone, and Diane Fry an outsider from the city. Diane is definitely out of her comfort zone in the countryside. And because she’s from Birmingham and has a distinctively different accent, local people can spot she’s “not from round here” as soon as she opens her mouth. As a result, Ben and Diane see Derbyshire and its inhabitants in totally different ways.

I did have a male and female duo. But a bit of gender reversal made Ben the caring, sensitive one, while Diane is more hard-edged and aggressive. This made them more challenging to write about, and it’s turned them into people who can still surprise me.

But that was the limit of my knowledge. Their personalities, their backgrounds, their families – everything else about them I discovered during the writing process.

I remember writing a scene in ‘Black Dog’ where Cooper and Fry meet for the first time. Suddenly, there were sparks flying between them. I didn’t expect that! Over the years their interaction has become a subject readers write to me about a lot. But I didn’t plan this relationship, and I don’t control it now. Ben and Diane decide what they want to do for themselves. And, like all of us, they’re quite capable of making disastrous mistakes in their private lives.

As we approach the 12th book in the series, my own relationship with these characters remains one of continual discovery. They’re old friends, but I’m still learning new things about them.

Of course, I believe in Ben, Diane and their E Division colleagues as real people. In fact, I rely on them to do their jobs as police officers and discover the story for me in each book.

But my aim has always been to create believable characters. I want readers to feel they could walk into a police station in Derbyshire and meet Ben Cooper himself. If that ever does happen to you, by the way, please say ‘hello’ to him from me!

Dead and Buried, the atmospheric new thriller starring Ben Cooper and Diane Fry, is available now.

Click here to find out more.