Sixteenth-century Spanish conquistadors discover the fountain of youth in Farnsworth’s thriller The Eternal World.

You’ve written about vampires who work for American presidents in your Nathaniel Cade series, and immortal conquistadors. What draws you into these worlds?

I’ve always loved the idea that all our myths and legends are true, but we’ve ignored or forgotten the reality behind them. My favorite stories are ones where the characters discover that the everyday world is only a mask for something stranger and deeper going on behind the scenes. Most of my ideas start by imagining how ordinary people would react if they suddenly got a glimpse of all the weird things behind the curtain. Then I set them all loose and see who survives.

Why the fascination with immortality?

I think it’s pretty simple: nobody really wants to die, but we’ve all got to face it. It’s like that great line from Unforgiven: “Kid, we’ve all got it coming.” That’s the moral of every story, all the way back to Gilgamesh. How we deal with death is a big part of what makes us who we are. But without those nagging reminders of a finish line in the distance, I think we’d become less human. Put another way, I’m not sure it’s possible to have a happy ending if nothing ever ends.

One character laments people must have something to live for. What would you live for?

Right now, I live for my wife and daughters. I know it’s a cliché: everyone says family is the most important thing. But I’ve found it’s hard to actually put that into practice. I have to make an effort, every day, at being a better husband and father, and putting my wife and daughters first. That’s one reason, in the book, the immortals are forbidden to have families: it changes your idea of what’s really important, or at least it should.

Science is realistically explored in The Eternal World. How much research did you do?

Jonathan Weiner wrote a fantastic book called Long for This World, about Aubrey De Grey and other scientists who believe that we’re within a century of functional immortality. That was a huge springboard.

Will we see more vampires or a sequel to The Eternal World?

I’ve no plans for a sequel, and my vampire, Nathaniel Cade, is on vacation. But we are talking about immortals here. They have a way of coming back, over and over again. So we’ll see.

What’s next?

Reader will be out in early 2016 from William Morrow. It’s about an ex-soldier who can read minds.