The biggest challenge Rian Johnson faced in his science-fiction thriller "Looper" wasn't to convince audiences that Joseph Gordon-Levitt would become Bruce Willis 30 years in the future. It was to make a time-travel movie that shuttles backward and forward while always moving ahead.

"I wanted to construct it so that time travel set up the situation, but then got out of the way," Johnson said. "Time travel doesn't exist in the film's present -- it's just in the future, so these characters have to deal with what it flings at them."

Gordon-Levitt and Willis play the same character, a mob hit man named Joe, in his 20s and 50s. The younger man flubs his assignment to kill his older self, setting off a tense pursuit pitting strength and stamina against experience and guile, with many lives in the balance.

The 39-year-old writer/director has long been attracted to dark themes. A 1996 graduate of the University of Southern California film program, he broke into features editing "May," a shocker about a young seamstress creating "the perfect friend" by combining her victims' body parts. In 2005 he merged the world of detectives and California high schoolers with the murder mystery "Brick," starring Gordon-Levitt.

He has also directed for TV, helming a recent episode of "Breaking Bad" in which meth lord and family man Walter White (Bryan Cranston) celebrates his 51st birthday in sociopathic fashion.

In "Looper," Johnson's most ambitious outing by far, making time travel seem plausible was easier than making its emotional repercussions believable.

"The old chestnut is true," he said. "Any kind of movie you're doing, 99 percent of the job is casting. I got really lucky with this one."

Johnson reunited with his friend Gordon-Levitt, and recruited Willis, a rare star who can balance action and drama. All the other main roles are cast against type, with Gordon-Levitt playing a dead-eyed killer, Jeff Daniels playing his blandly sadistic mafia boss and Emily Blunt playing a Kansas farm girl.

Blunt threw herself so forcefully into a scene where she wields an ax that she injured a shoulder, completing the film in considerable pain. Pierce Gagnon, the 5-year-old who played rivetingly intense scenes as her son, was a trouper as well, Johnson said.

"When the cameras were off, he'd be a normal 5-year-old, hitting people in the shins with his sword. When the cameras were on, he would hold his own against Emily and Joe."

That emphasis on acting is unusual for science fiction, where visual effects often trump cardboard acting.

"It was very important for me that this was a character-based movie, that the payoff was not this intricate puzzle of time travel resolving itself but more about the emotional place that it brought you to. These characters are dealing with a very hard moral situation. It's about the moral compass swinging back and forth and at the end, who the audience is going to be pulling for."

Key segments of the film are set in a prosperous, glossy Shanghai, although Johnson's first draft imagined them in Paris. The location changed when Chinese financiers signed on to co-produce the film. When it is shown there, additional Chinese scenes will be included.

Johnson said he believes that U.S. audiences will accept the episodes and not see them as a nationalistic form of product placement.

"The reason I was comfortable doing it was originally they went to France. But we did not have the money to go to Paris to shoot and we faced the prospect of faking Paris in New Orleans. I didn't adjust a single thing content-wise other than changing it from France to Shanghai, because we could actually go to Shanghai and shoot. I didn't change it to make it more appealing to a Chinese audience. I thought, 'This actually might be a little cooler for the story.'"

The Asian setting makes for a starker contrast with the U.S. city and foreshadows the climax, which echoes the dark Japanese anime epic "Akira," Johnson said.

"As a filmmaker I didn't feel uncomfortable. I felt the opposite of uncomfortable. Shanghai as the destination of the future feels a little more in keeping with that sci-fi, 'Blade Runner' tradition."

Colin Covert • 612-673-7186 • On Twitter: @ColinCovert