Two years ago, Morgan Spurlock rang Abercrombie & Fitch to offer them his services as a model. The response wasn't wholly positive. "Have you looked in the mirror?" the company's press agent asked. "You're pale. You're out of shape. You're not very good-looking. You have a moustache. You're going bald." That offending 'tache shakes with laughter at the memory. "She broke down every flaw for me. She said I was an ugly person. She was very, very offended by my offer."

So Spurlock – pale skin quickly thickening – returned to his phone book and called more companies hoping to cut a deal; 600 in all, of which he scored with 15. This 2% success rate left him with a "tremendous sense of despair", a heightened empathy for call centre workers and, finally, a film: POM Wonderful Presents: The Greatest Movie Ever Sold (though sadly the cameras weren't rolling when Abercrombie slapped him down).

An examination of how films are funded by product placement, and paid for in exactly this way, The Greatest Movie is a doddle to flog, especially for such an expert salesman. The crowds at the Sheffield Doc/Fest, where we meet in June, lap up his Barnum-style charm; putty in his light-touch jazz hands. Spurlock is a breath of confident showmanship in a genre not notorious for it, just as The Greatest Movie is light relief amid the tub-thumping and brow-furrowing. It's more intricately gimmicky than McDonalds-for-a-month experiment Super Size Me (2004), the film that introduced us to Spurlock's brand of human guinea-piggery. And more tone-appropriate than his second, the bounty-hunting travelogue Where in the World is Osama bin Laden? (2008), the critical mauling of which seemed likely, for a time, to spell the end of his big-screen career (he hasn't read his press since).

Morgan Spurlock discusses his latest film, Pom Wonderful Presents The Greatest Movie Ever Sold at the Sheffield Doc/Fest guardian.co.uk

The Greatest Movie was dreamed up in January 2009, but it took nine months of ring-rounds before anyone took the bait. "There were countless times when we were just like: fuck, what if nobody comes on board? There was no plan B." Then, the first handshake: vaguely eco roll-on deodorant Ban. Then Sheetz, the petrol station chain, then a dribble more interest, then the hotel chain Hyatt, and, eventually, the big kill: a headline sponsor, who paid $1m in exchange for their brand running above the title. In fact POM, a pomegranate juice outfit that was 39th on Spurlock's list of drinks, has more than paid their way. For it's they who've unexpectedly added spice to the mix: since the film's release, POM's claims over the cardiovascular and prostate benefits of concerted swigging have been the subject of action by the US Federal Trade Commission, which POM is challenging. ("If I were to drink this for a year," says Spurlock, "I'd get the greatest erection ever sold.")

What Spurlock wanted was more of this kind of controversy. "We tried to get the shittiest people to give us money. The worst corporations. Gun manufacturers. I thought: we gotta get rifles in the movie, things that actually kill people. I called cigarette companies. I called BP. They wanted nothing to do with it. There was a real ethical conversation to be had about where to draw the line. It's a shame we couldn't do it."

What one is left with is a film that worries at the limits of personal integrity. Spurlock now believes the moment art hops into bed with commerce "there's a 100% chance the content will get corrupted". For cash-strapped film-makers, he thinks the only route forward is to shoot commercials for cash and then self-fund your documentaries. It's hardly a new dilemma. "Art has had sponsors for centuries. People would have their work subsidised, whether by the rich or the church, and then take the money and then go off and fight the man on the side. Now I've got all these commercial offers flooding in, and the question is: can I take that money and then go and blast the doors open on other types of arena?"

To some extent, the dilemma that emerges most pressingly from The Greatest Movie is not one for the artist but the consumer. "Making this film has made me infinitely more aware of how I want to raise my kid," says Spurlock. "It's made me much more cognisant of where I want to spend my money. If you wanna live in a box your whole life then maybe you shouldn't see the movie."

Part of the sell of the film is its interactivity: getting involved with it, in any capacity, is an extension of the experiment itself. When you see it, when you read these words, you'll be fulfilling some of those obligations (ticket stubs sold, media impressions managed) demanded by the investors. On some level you're as implicated as those residents of Altoona, Pennsylvania, who took $25,000 to rename their city POM Wonderful Presents: The Greatest Movie Ever Sold, PA for two months this summer.

It also raises intriguing teases about to what extent people – in their professional and personal lives – are reliant on their own brand (Spurlock's is professionally analysed in the hope that'll match him more closely with potential investors, and comes back as "mindful/playful"). When he asks people on the street to deconstruct their USP, they're acute and funny; but they don't take it seriously. Should we? Ought people to have a more accurate sense of commodity value?

Spurlock takes a holistic approach. "I think your sense of self should be your commodity value – period. You're marketing yourself every day based on that self-worth." Isn't that different: what you produce, rather than what you are? "But it's an extension of who you are, of your sense of self. Whether you stand for quality or value or truth."

With Spurlock, of course, the separation of self and occupation is an unusually sticky one. His personality has long been his wares to hawk; right from his first break hosting an MTV dare show in which punters would scoff unsavouries for cash (full jar of mayo = $235, worm burrito = $265). So perhaps it's natural he tends to overestimate the connection for others.

"My father was an entrepreneur," he says. "And he was all about hard work and saying what you mean and meaning what you say. He never had a contract; if he shook your hand, that was all you're needed. And that as a person bled into him as a businessman."

Spurlock is the same: an honest, average Joe. But he's also canny enough to know that being a human guinea pig, no matter how high-falutin' the experiment, isn't something you can necessarily keep doing into middle age (he's 40). Spurlock's shtick – a consumer superhero who confronts people we all might if we had the gumption – either needs to toughen up to enable him to take on harder targets, or to adapt. The former isn't going to happen: Spurlock is a natural cheerleader who likes being liked and feels at home in the mainstream (upcoming documentary subjects are the comic convention Comic-Con and sports agents).

But that's not to say he's not without serious artistic ambition. The goal of all documentarians, he says, is "to get their film into the cinema". Digital release may ensure an audience, but "is it the best-case scenario for you as a film-maker? Of course not. Is that what you wanted? No."

And Spurlock would, as it happens, like to make the move into fiction – when we chat in June, he talks about "getting his feet wet with a $10m project … of course I would love to do that. You'd know how the story ends, which is just the best thing ever. You would have a finite shoot period. You wouldn't just have to keep on shooting until you stop." He enthuses about the Peter Jackson model – going from Heavenly Creatures straight to Lord of the Rings.

At the time, it sounded like a pipe dream; since then, details have emerged of Little Green Men, an adaptation of the Christopher Buckley novel about a political talkshow host who is abducted by aliens, on which Spurlock seems to be installed in the director's chair. It'll be fascinating to see whether he pulls it off, and without resorting to product placement. Pepto-Bismol Presents: Little Green Men, anyone?