When Clio Barnard's film The Selfish Giant was unveiled triumphantly at the Cannes Directors' Fortnight last May, more than a few critics bandied around words such as "naturalism" or "neo-realism". The grim, touching story of two boys trying to scrape a living on the outskirts of Bradford, Barnard's picture follows in the tradition of earlier tough classics such as Kes and Nil by Mouth. The performances are rough and unshowy. The story is loosely structured. But Barnard is wary of the realist delusion.

“I guess I got interested in this expectation that film can deliver reality – which is a nonsense,” she says. “I was trained, as a visual artist, to be rigorous about that. The founders of the Direct Cinema movement would say this is more real because now we move the camera. That always seemed like a false aspiration to me.”

Cinema needs clear thinkers like Bernard. A tall, thin Yorkshire woman, she easily zips from critical analysis to playful quips in the space of a friendly clause. It is not altogether surprising to learn that she came to film through the world of the visual arts. She has the seriousness of purpose that such training often instils. The daughter of an academic, she seems to have spent early years in the ideal environment for a future artist.

“I grew up on the edge of the moors,” she says. “It was romantic when the weather was good. It was bleak and beautiful. I really felt a great connection with the landscape.”

That comes through in The Selfish Giant. There are some extraordinarily bewitching shots of superficially unattractive vistas.

“Yes, my favourite shot is the one with the cooling towers and the sheep. They seem benign and beautiful to me. We could have made that sinister. But we chose to go the other way.”

Bernard eventually made her way to art school in Leeds where – if she isn’t being modest – she tried hard to paint, but couldn’t quite get her head around the oils. Then an original style of stop-motion animation began to work its way organically into her work. Using a hand-cranked Bolex movie camera, she began taking images of her work and, without really willing it, Bernard found herself turning into a film-maker.

“I made an experimental film and did some work for galleries,” she says. “I also made a short narrative film and developed a feature that didn’t get made. Then I went back to the National Film School because I thought if I am going to make narrative film, then I had better learn the language. I could think about framing, but I needed to learn how to work with actors.”

All of which makes sense. But, when Bernard's first feature emerged, it displayed few of the conventional techniques of narrative film-making. Among the most original and celebrated films of 2010, The Arbor placed the actual voices of inhabitants from Bradford's Buttershaw Estate in the mouths of actors as it sought to investigate the short life of local playwright Andrea Dunbar. Again, she returns to that notion of the illusion of "reality" in film.

“I wondered what would happen if you had documentary sound and totally constructed images,” she says. “I was trying to question that idea of film being able to tell the truth. I can’t tell the truth about Andrea’s life in 90 minutes. How could I?”

While shooting the film on Buttershaw, Barnard happened upon the two young men who would inspire the story of The Selfish Giant. Matty and Michael, troublesome and charismatic, scraped a living by (among other things) scavenging metal to sell to local dealers. The seeds of a story began to germinate in her mind.

“I had done this workshop in the school and Matty walked in with this attitude,” she says. “The whole atmosphere of the room changed. He strutted around for five minutes and then was off. I walked down to the Arbor, where the film was shot, and he was wearing all these clothes you need for ‘scrapping’: rigger boots, really dirty clothes. But he had attitude. People would call him a ‘pikey’ and he’d deliberately ramp up that look: a ‘fuck you’ to them. I admired him for that and I admired him for his loyalty to his best friend.”

The film follows the waxing relationship between a hyperactive, consistently energised young tyke called (in tribute to the film’s origins) Arbor and his largely, slower, more responsible pal, Swifty. Over the course of an impressively unsentimental film, they dally with informal horse racing, get themselves excluded from school and fall in with a particularly dodgy scrap-metal merchant.

There are bits of Steinbeck's Of Mice and Men in here. Barnard admits that she thought of Midnight Cowboy while making the picture. But the connections with Oscar Wilde's great fairy story The Selfish Giant are more difficult to disentangle. Clearly, Kitten, the metal merchant, is the Giant. Not much else is clear. She and her resourceful producer, our own Tracy O'Riordan, had many discussions about the issue.

“We all had that discussion: is this the right title?” she says. “The original script was written from the Giant’s point of view. So, the connection was then much clearer. But, after I had finished that draft, I realised that the characters I most cared about were the two boys. I was interested in what happened to their friendship when they came into conflict with him.”

After much chatter with O’Riordan and their editors, Barnard was eventually convinced she should stay with the original title. This is the right decision. The sadness of Wilde’s story remains palpable in Barnard’s more hard-edged film.

“It is still a platonic love story,” she says. “It is, I hope, still a realist fable. That was another reason for keeping the title. There is redemption. It uses metaphor. Are those things always in fables? Maybe I am undermining my own argument here. Ha ha!”

Like so much of the best (if she'll allow the phrase) naturalistic cinema, The Selfish Giant features superb performances from nonprofessional child actors. Ken Loach built Kes around such a turn. A few decades ago, Iranian film-makers constructed a new national cinema from films about children. Conner Chapman (exhaustingly fidgety as Arbor) and Sean Gilder (touchingly still as Swifty) make the film their own. Consistently excluded from what remains of society, they are forced to construct their own improvised alternatives.

What’s the trick to directing those performances?

“Get a great casting director,” she laughs. “What both children have in common is a very strong instinct for story. Conner is actually quite reserved, but once he’s telling you a story – whether it’s all true or not – he is completely absorbed in it. It was the same on set. He is actually a rigorous young actor.”

Director of two heavily acclaimed pictures, Barnard now has to decide how far into the mainstream she wishes to lunge. Not yet prepared to embrace Hollywood, she next embarks on a version of Rose Tremain's recent novel Trespass. Adaptation is a whole new adventure. Is she daunted?

“At first I thought, ‘this is great’. The story’s already there,” she laughs. “But actually it’s so hard and I am at the really hard bit now.”

I shouldn’t worry. She doesn’t strike you as the type to give up.