There was a time when I thought a “sleeper” was a thick plank holding a railway track together. Now I know it is also a movie that opens in a yawning auditorium of empty seats that gradually fill up over the weeks as word gets around until it's the hit of the season, the decade and beyond.

The latest big surprise, Kenny, comes from Down Under. Kenny is an Australian “poo specialist” and part philosopher, part humanist, part humorist and all heart. We discover these loveable traits throughout this hilarious film about the life and times of this specialist in Portaloo maintenance. Our Everyman hero endures a monster-truck chase, fire-throwing rowdies, indignant posh ladies, neurotic employees, a dad with “two cents in his emotional bank account” and much more, while working for a company that provides mobile loos for every occasion, from stock-car racing to the Melbourne Cup.

As I watched, totally entranced, I thought I was watching the funniest and most profound documentary I'd ever seen. It has all the ingredients of the perfect sleeper – quirky, very original, something you can't wait to tell your friends to see. But when I read the end credits, I realised that the main characters were all actors (working on a potentially Oscar-winning script). Three cheers for the Jacobson family who wrote, directed and acted it.

Kenny isn’t the first film to build an audience slowly. When I attended the premiere of The Red Shoes one bleak day in September 1948, I was one of a handful of balletomanes in an otherwise empty cinema. But over the years The Red Shoes has appealed to ballet lovers worldwide.

How many sleeper hits can you actually name? Surprisingly few, I’ll bet. I'll also wager you'll be surprised how many you have seen without even knowing it. One of my favourite gross-out sleepers is There’s Something About Mary. The 40-Year-Old Virgin is another sleeper whose toilet humour gave it an audience sniggering at our sexual misfit as he tests a box of condoms by either stretching them beyond endurance or blowing them up like balloons. Sophisticated it ain’t.

Middle American settings populated by odd characters, are sleeper favourites. Garden State is written and directed by Zach Braff (of TV’s Scrubs), who plays a 26-year-old son returning home for the first time since high school to bury his mother. This update on The Graduate about the uneasy rite of passage to adulthood, is – to a British sensibility – rather more about immature slackers who are terminally depressed.

To American audiences, though, it's hysterically funny, a real-life recognisable dilemma in which high school buddies negotiate an incomprehensible larger world and find the appropriate response is lethargy, mind-numbing drugs, a mighty roar into a dreadful abyss and (of course) true love.

A sleeper hit seems to happen when nerdiness migrates to the mainstream – it is the thing that links 28 Days Later, a British-made sci-fi sleeper, to La Femme Nikita from France. The former is a futuristic horror movie about a virulent strain of rage infecting the populace and turning them into flesh-eating zombie berserkers; the latter is about the assassin training of a young girl blackmailed into serving a counter-terrorist cadre.

And there’s something more to the magic formula. Little Miss Sunshine had a mixed bag of misfits from a sassy little girl to a sexy old grandpappy, and some politically incorrect behaviour that was mildly shocking. Mean Girls showed how a plain Jane with a heart of gold is turned into a bitchy high school horror and back again. Rocky tells of how a battling underdog comes out on top.

Maybe that’s it: every sleeper hit has a distinctive underdog, a very particular hero or antihero to root for. Is this a reaction to the superhero flying high in the sky, altogether out of reach? Is this perhaps the sleepy majority’s celebration of the Supernerd? All I know is I can’t wait to get hold of my new Kenny T-shirt.

Kenny is released on Sept 28 , 2007

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