Cavanagh’s fourth Eddie Flynn thriller, Thirteen (Flatiron, Aug.), pits con man turned attorney Flynn against Joshua Kane, a serial killer who is not on trial but is on a jury.

Why, as someone from Belfast, Northern Ireland, did you choose to set your series in New York City?

When I was reading Lee Child, I found out that he was from Birmingham and I didn’t know he was English. John Connelly was another one of my heroes, and John’s from Dublin and he sets all his thrillers in America. And I didn’t know you could do that. It never occurred to me. I thought “Wow! They could do that, I wonder can I do that?” The other thing was that my first books were all legal thrillers, and so, if I’m writing a legal thriller set in Ireland or the U.K., we have solicitors and barristers. The barristers wear horsehair wigs and gowns. The type of breakneck thriller that I wanted to write didn’t suit a) having two main characters and b) one of those characters wearing a wig. It’s very hard to write a cool thriller when one of your characters wears a wig and is terribly polite, and I thought, well that doesn’t work. Whereas if I set it in the U.S., it’s much easier—there’s only one lawyer. Plus, New York City seemed like the right place, the pace of that city seems to fit with the pace of the books. I thought, “That’s a natural fit, I’ll try to do it there.”

What sort of research did you do in preparation for the writing of Thirteen?

I did a lot of research into serial killers, which was fun. My internet search history would get me arrested in any jurisdiction. I wanted to create someone who had a sort of physical defect that fed into their psychology, and that was how

I came up with Joshua Kane. He has a particular condition called congenital analgesia, which is a real thing. I thought, what would that do to someone if you’ve never felt pain, never understood it, would you not be slightly fascinated by it?

How do you go about creating a character like Joshua Kane?

I had to look at Kane as if he were the hero of this book in the chapters devoted to him. I had to think of him in terms of what he wanted. I had to treat him as a real person with real goals, but his goals obviously are dastardly and in complete opposition to those of Flynn. I think if you’re going to create a really bad villain there almost has to be an element of well, I’m almost rooting for him but yeah, he’s doing something really bad, so I don’t want to root for him.