The pseudonymous Swedish couple who write as Lars Kepler pit their independent-minded detective, Joona Linna, against weapons dealers in The Nightmare.

What intrigued you about international arms dealing?

Our idea started with seeing Sweden as the perfect country for large-scale arms smuggling. Sweden hasn’t fought a war since the Napoleonic wars in 1814, but has the biggest export of arms per capita in the world. We thought it would be exciting to let three young people on a pleasure boat trip suddenly realize they’re the targets of a professional killer. While they’re trying to escape the killer in the Stockholm archipelago, Det. Insp. Joona Linna investigates a strange death: the general director of a Swedish weapons committee is found hanged in his apartment. This case turns out to be connected to the boat, and the investigation rapidly turns to arms smuggling and dirty politicians. When we started to write, we were thinking about The Bourne Identity and movies like Three Days of the Condor.

What inspired you to have Linna suffer from debilitating migraines?

We were in need of a hero and wanted to avoid the cliché of a divorced, unshaven detective with a whiskey glass in his hand. Joona has these horrible migraines as a scar from his dark past. A past that, unfortunately, is catching up with him very fast.

Why did you decide to make Linna a Finn rather than a Swede?

Finns are the biggest minority in Sweden, more than 5% of the whole population. We thought it was time for a Finnish hero in Swedish fiction, and we really liked the fact that he was a bit of a stranger in Sweden, someone who is both inside and outside of the culture. Finns are considered serious men of few words (when sober). We decided to keep some of the clichés and turn others upside down.

Is it challenging to maintain a suspenseful plot and consistent characters with two writers working on the same material?

It’s the best thing we’ve ever done. This kind of immediate input is very satisfying and creative. We’re always discussing the plot and our characters. Our friends and families probably get sick of us sometimes. We continue to talk about our story when we cook, when we are shopping for dinner, and when we pick up the kids at school. If we have a good idea in the middle of the night, we wake each other up. And when we write, we swap texts maybe 20 times a day. We rewrite over and over again. After a while we don’t even know who wrote what in the first place. That’s when Lars Kepler has taken over.