After having the idea about time-travelling assassins, Rian Johnson then got Bruce Willis and Joseph Gordon-Levitt to play two versions of the same hitman Rian Johnson likes breaking cycles, tearing up patterns. His first film, Brick, looked like a high-school movie, but spoke like a detective story. It was Dashiell Hammett enrolling at Sweet Valley High.

"I go to unexpected places," says Johnson, who's buzzing around a hotel room in uptown Toronto, hyped up on the rave reviews that have greeted his new film, Looper, which just opened the city's film festival. "Going outside of the genre you're working in always seemed like a good way of infusing it with some new jazz."

Johnson's jazz blows wild and free right from Looper's first note. It opens with a crescendo: three tense minutes that stake its claim as one of the most arresting films of the year. A man stands in the Kansas sun. A shotgun on his arm, a white sheet on the ground. He checks his watch, grimaces and waits. Checks his watch. Waits. Then waits some more. A figure bound, gagged and blindfolded appears on the sheet. The man fires without hesitation, squints at the body and wearily starts towards it. The sequence starts again, with a new victim. Watch, wait, grimace, shoot.

The man is a Looper: an assassin hired by gangsters to kill whoever appears, no questions asked. His targets are enemies of a future mob, sent back 30 years from 2074 to be murdered decades away from detection. It's an incredible premise, but it results in a pedestrian life. A Looper is the fast-food worker of cold-blooded murder. The same old slaughter day in, day out.

This cycle is broken when a Looper called Joe (played by Brick star Joseph Gordon-Levitt) comes face-to-face with a target who won't just kneel there, whimper and die himself. Older Joe (Bruce Willis) is back to fix the past into a shape that suits his present. When he escapes it's up to Joe junior to track him down and kill him, to "close the loop.”

"Sci-fi is good at using these magical concepts to address crucial human needs and emotions," says Johnson. "Looper is about that brash over-confidence of youth hitting against the world-weary over-confidence of age. Joe's older self is no less delusional in the way he thinks he has it all figured out."

Johnson speaks in quick, clipped sentences. He pronounces "sci-fi", "SCI-fi" as if to tip the balance in the genre's favour. He's bespectacled, a little intense, but he's too gregarious to be an old school geek, too friendly to be a fanboy.

He wrote Looper 10 years ago as a short ("a little three-page thing") when he was bored of waiting for Brick to get financed. It was supposed to be a hobby project, shot on weekends with a few friends. Then, when he fancied working with Gordon-Levitt again, the premise was expanded, fleshed out into a world embellished with sci-fi flourishes (drugs taken through the eyes, the human race's evolution of weak telekinetic powers). Still, the ingenuity of the original idea the three minutes that form that opening remained.

It shouldn't be too much of a surprise to hear that Johnson was reading a lot of Philip K Dick as he wrote the film, but he says his main influence was less obvious: Japanese author Haruki Murakami and his 1985 fantasy novel Hard-Boiled Wonderland At The End Of The World had a strong bearing on how Looper played out.

"His characters have this new-wave cool to them," says Johnson. "There's also a casualness with which he introduces sci-fi elements that appealed to me. Like the way we integrate the telekinesis stuff. They [the Loopers] use it as a bar trick to pick up chicks. Normally that would be this huge sci-fi idea. I took that technique from Murakami."

'Murakami's characters have this new-wave cool. The casualness with which he introduces sci-fi elements also appeals to me'

Johnson has crammed Looper with these subtle touches. It's set in and around Kansas City 2044, but the future looks, frankly, knackered. The economy has fallen through the floor. The homeless line the streets. The car of the future is a Ford Fiesta with solar panels gaffer-taped to the roof (a nod to Cuba's rusting "Yank tanks", says Johnson). We've headed out of the bustling metropolis of Blade Runner into a backwater, more easily twinned with the scrapyard slums of District 9 than the sleek, centralised hub of a Spielberg-ian future. As Joe remarks of his boss, a 2074 expat played by Jeff Daniels with the tired air of a murder middle manager: "He runs the city. In any other city that would mean something."

This decrepitude leaves the well-paid Loopers looking like a future echo of the wiseguys in Grease; big fish flapping their fins over the edge of a very small pond.

"They're given these guns where if you generally aim at something it's going to hit it within 15 yards," he says. "It's meant so that the hungover Looper is likely going to hit the guy."

Listening to Johnson describe the most iconic characters he's created as "•••••••" it's tempting to believe that he enjoys undermining his creations' cool, or at least subverting the genre he's working in by double-crossing our expectations. He's done it before. Brendan, the supposedly hard-knuckled private eye of Brick, is a bit of a sap, constantly taking his glasses off so the bigger boys can knock him down. In Looper, both Young and Old Joe look like heroes, but they're (literally) too self-involved to learn anything from life, until they're forced to.

"I drew his arc while looking at Bogart in Casablanca," says Johnson of Young Joe. "He starts off as a selfish, isolationist who's not willing to stick his neck out for anybody. By the end of the film he gets around to a selfless act."

We'll keep schtum about Joe's route to redemption; mapping it out would spoil the film, but it should be said that after assuring us we're watching a simple sci-fi/thriller/chase movie, Johnson pulls us to a halt. He decides to add romance and drama into the mix. We're introduced to Sara (Emily Blunt), a farmer living out on the plains with her son. Sara offers Joe a hideout away from his marauding older self. The introduction of the rural setting and a set of dependents allows Johnson to click down the gears and show us Joe growing up a little. It's not really a lull, more a breather before a further shocking twist that shows in dramatic detail a crime that few arthouse movies would dare to tackle.

"The audience has whole a set of expectations when they see Bruce Willis show up in a movie," Johnson says. "Like, he's going to be the hero who'll find the bad guy and fix this. So the fact that momentum would play back against itself when that moral turn happened was really exciting to me."

'They're doing these horrible things in order to protect what's theirs. That's the reason that horrible things are done in the world'

If Bruce isn't really Looper's baddie, it's only because black-and-white morality doesn't have a place in Johnson's world. Is Sara a mother who's willing to kill to protect her son a better person than Old Joe, a man who'll kill to save a life that's increasingly distant? Is it foolhardy of the younger Joe to hang on to the life he knows, even when the future is warning him against it? These questions tied up in the narrative trickiness of time travel are likely to get the fan forums bubbling. But for Johnson, they're simple cases of characters acting out of self-preservation.

"They're doing these horrible things in order to protect what's theirs," he says. "That's the reason that horrible things are done in the world."

Given that Willis's character is a stubborn, ruthless grouch, can we see Looper as a fear of ageing, too? Less that than a fear of getting trapped in a pattern, reckons Johnson. Old Joe is repeating himself. He thinks he's surpassed his idiotic younger self, but he's using the same tool violence to dig himself further into the rut.

"I guess that's our job as human beings," says Johnson. "To figure out where those unhealthy cycles are and break them."

For Johnson, that means switching up the work quickly and often, following stints as a TV director (he helmed two episodes of Breaking Bad last year) with time out singing and playing banjo with his folk band the Preserves, or directing the esoteric shorts which turn up as easter eggs on his films' DVD releases.

Looper is full of people protecting what they know, saving what's theirs by making the same mistakes over and over. That's what Rian Johnson's career is pushing against. He doesn't want to give us what we know; doesn't fancy repeating himself, either. He's not keen on Looper being labelled a sci-fi or thriller or romantic drama, any more than he wanted Brick called film noir. Rian Johnson likes breaking cycles, tearing up patterns.