Aside from being the best-selling author of 16 books, Nicholas Sparks has a hand in the hit movies based on his stories.

Opening today, “The Lucky One” is the seventh in a string of cinematic romances that include “The Notebook,” “Dear John” and “The Last Song.” Though they haven’t received extensive critical acclaim, the financially successful movies continue to draw in passionate fans on DVD, cable and streaming.

“The Lucky One” is about Logan (Zac Efron), a Marine stationed in Iraq who discovers a lost photograph that seemingly saves his life during a bombing raid. Once home, on a search for the girl in the photo, he finds Beth (Taylor Schilling), who runs an animal shelter and who lost her brother in the war.

Logan is based on real-life military men Sparks knew, including, specifically, a chaplain whose job was to hold dying soldiers on the battlefield, but also teens who go to war fresh out of high school.

“I coach track and field in my hometown,” Sparks said during a recent visit to San Francisco. “I know these guys for four years. Some join the military, and they come back profoundly changed. They’re only a year older.”

While Sparks designs his stories for greater emotional, rather than intellectual, impact, his process comes down to three considerations.

First, he walks the fine line between the familiar and the cliché. Next, there’s the line between drama and melodrama, and third — perhaps most challenging — is the line between genuine emotion and manipulation.

He explains that if Logan and Beth kiss and the audience responds, “It’s about time!” — that’s emotion. But if they kiss out of the blue, it’s manipulation.

“Some people might say that you went overboard here or there. But I’m always 100 percent conscious of it,” he adds.

Despite being established and wildly successful, Sparks still has “ups and downs” when he writes: “There are moments I read it and go, ‘That’s great,’ and then the next moment, ‘That’s just terrible.’ It’s part of it,” he says.

Writing, he says, is easy — but writing well? “There are very few things in life that are harder to do,” he says. “It keeps me up at night sometimes.”

IF YOU GO

The Lucky One

Starring Zac Efron, Taylor Schilling, Blythe Danner, Jay R. Ferguson

Written by Will Fetters      

Directed by Scott Hicks

Rated PG-13      

Running time 1 hour 41 minutes