When Craig Johnson began working on his first Walt Longmire mystery novel in 2003, he had several reservations about writing a series. One of his biggest concerns was that a series might become too formulaic or boring.

“At the start, the idea of doing this for the rest of my life was pretty daunting,” Johnson said. “But Viking-Penguin is a great literary publisher, and they let me do just about whatever I want.”

Fifteen years and 14 Longmire novels later, Johnson’s best-selling books about a Vietnam-veteran-turned-sheriff of fictional Absaroka County, Wyoming, have sold more than 2.7 million copies worldwide. They also spawned a televised adaptation, “Longmire,” which aired for six seasons from 2012 to 2017.

Walt’s flawed and multi-faceted personality has allowed Johnson to explore many different issues and aspects of the character. The sheriff has a core of decency, integrity and honor that he rarely violates, but at the same time he struggles with his depression, age, weight and the challenges of carrying out his job in a small, tight-knit rural community.

“I think the key element to any series of books with a singular protagonist is letting the character breathe and develop and evolve. Otherwise you’re in a widget factory stamping out sausages and it gets tiresome for readers to read and writers to write,” Johnson said.

The author takes the evolution of Walt’s character to a whole new level in his latest installment in the series, “Depth of Winter,” which he will discuss at two events Sunday in Sunriver.

Rather than the usual Longmire mystery set in a small Western town with the middle-aged sheriff using low-tech crime solving techniques, the plot of “Depth of Winter” could have been pulled from the script of a Hollywood action film.

In the story, Walt heads into the Mexican desert alone to try to rescue his daughter, Cady, who was kidnapped in Johnson’s last book, “The Western Star,” by Walt’s longtime nemesis, the head of a vicious Mexican drug cartel, Tomás Bidarte.

“This one’s a bit more epic than the previous books and more of a thriller than a mystery,” Johnson said. “Each book is kind of like a mountain you’ve got to climb, and this one has been building for about five books.”

While this thriller is set in a different locale and takes some darker turns than previous Longmire novels, Walt is not suddenly transformed into a Jack Reacher-style anti-hero. Walt’s basic decency and the new cast of misfits he recruits to help him on his potentially suicidal mission, plus Johnson’s witty dialogue and plot twists, all stay true to the Longmire legacy.

“Walt has almost abandoned the thought that he’s going to survive this situation, and he’s building probably the worst team in the world, but that’s telling of Walt,” Johnson said. “It’s just something he’s got to do to save his daughter. So while this may start out as the idea of a Hollywood big-budget epic, reality quickly intrudes. For me, that’s one of the joys of writing the books — they let me do things that are unexpected, insightful and interesting.”

Despite his worldwide success and literary fame, Johnson still lives and writes on his horse ranch near the tiny town of Ucross, Wyoming (population 25). And he carves time out of his busy writing and publicity schedule to visit small, independent booksellers such as Deon Stonehouse of Sunriver Books & Music, who supported him early in his writing career.

“I remember the first event I had in Sunriver for my first or second book and there was maybe half a dozen people,” Johnson said. “For me it’s like old-home week. I enjoy meeting and talking with people and letting them know what I’ve been working on and how this book came to be.”

Stonehouse says Johnson’s books are “by far” the best-sellers in her store and he is the biggest draw among author events she hosts. When Johnson visited Sunriver last summer, 660 fans attended the two events he headlined, and she expects a similar or even higher turnout on Sunday.

“I am gratified, grateful and amazed by his kindness and loyalty in coming back every year,” Stonehouse said of Johnson. “He’s a big deal now touring all around the world, and he still comes to our little store.”