Down in the depths of London's Cambridge Theatre, Tim Minchin is hammering away at a piano. Big, baroque phrases billow up the stairs, competing with the workmen who are still building the stage for the West End transfer of his critically-adored musical Matilda. Just a little daily practice? "No. I've never practised in my life," says the Australian comedian/musician, swivelling round on the piano stool. "Never. I don't have any discipline. I still can't do my major scales in both hands."
Of course he can't. Minchin is, among many other things, a professional rebel: the comedy nerd who raises the roof at the O2, performing songs about crying babies and cataracts with the dry-ice posturing of a rock star: the sweary scruffbag in an untucked dress shirt who packs out the Sydney Opera House and the Proms; the noisy atheist preacher who tours his God-prodding gags around America's Bible belt. In short, he thrives on getting into places he shouldn't be – never quite fitting in, but almost always selling out.
Today he has toned down his stage look – crazed eyeliner, backcombed hair, dissolute tails, bare feet – but he still looks comically out of place. His ginger hair has been slicked back and his eyes are nude behind thick-rimmed spectacles, but his waistcoat is still jaunty, his jeans skinny and his boots pointy. He looks like a rocker who has gone back to school. Which, in a way, is what he is.
Minchin's adaptation of Roald Dahl's classic about a schoolgirl with magical powers opened at the RSC last November to nightly standing ovations and reviews that called it the best British musical since Billy Elliot. Desperate to bask in Matilda's glow, the London Evening Standard has nominated it for three of its West End theatre awards before it has even opened. Not bad, says Minchin, for someone who can't read music, only got to grade-two piano and can just about sing in pitch – "which is the only thing you can say for my voice".
"Matilda is not an avant-garde, cutting-edge work of composition but emotionally it hits the marks," he says. "It makes people laugh and it makes people cry, which all theatre should do."This is typical Minchin, a man capable of screw-you grandstanding and crushing insecurity in the same sentence. He tells me that when the RSC offered him the job, his wife said: "Why don't they get someone proper to do it?" As it turned out, Minchin is a perfect match for the irreverent Dahl; his score is a little bit Beatles, a little bit bossa nova, with plenty of wordplay and hummable tunes. It has seen him, and playwright Dennis Kelly, hailed as the new Andrew Lloyd Webber and Tim Rice.
How did the self-styled "hack pianist" from Perth end up here? Six years ago, Minchin, now 36, was playing in pubs with his brother's band. From a musical family (his uncle is a bluegrass musician, his cousin is an opera singer and his siblings are all talented singers), he studied English and music at university and spent the next decade writing songs and trying to get a record deal. In 2005, he gathered all his comedy songs into a cabaret show. "To get it off my chest," he says. "So I could focus on my band, Rosencrantz."
Ah, Rosencrantz. The Tom Stoppard play remains Minchin's favourite (he is in talks to do it at the Sydney Theatre Company), but the band named after it never really took off. His cabaret did though, whisking him from Melbourne to Edinburgh where he won the Perrier Best Newcomer Award in 2005.
"I brought the idea," he says, "that if you want to be a musical comedian, you have to write proper songs." And he does – whether nine-minute jazz riffs about homeopathy and hypocrisy or short, sharp punk shocks on prejudice and the Pope. Over six years, he has grown into his act, his epic songs transferring effortlessly to the Reading Festival or the Royal Albert Hall, with accompaniment from the 55-piece Heritage Orchestra.
His look, meanwhile, started out as a visual joke – a preening rock god plays daft piano in tiny venues. (Russell Brand, for one, saw its potential. "All I know is that Russell came to see me at the Soho Theatre in 2006. I went back to Australia and when I came back he was huge and looked like me.") Now a bona fide stadium star, Minchin has his hair chemically straightened and wears blue contact lenses on stage. Somehow the joke still works.
He writes and rewrites obsessively, worrying about every beat, "like a puzzle". He claims never to listen to music – his iPod is full of science podcasts – and really, his songs are extended games, teasing hushed audiences to a slap-in-the-face conclusion. Nothing – and no one – is sacred. When, six years ago, a critic dared to give him a one-star review, he immortalised him in song: "I still wanna cut big chunks of flesh out of your stupid face/And make your children watch while I force you to eat them." Even a lullaby for his crying baby takes a turn for the sinister: "Papa's gonna buy you a mocking bird/In the hope you get avian flu."
One critic said that you leave a Minchin gig believing everything you ever believed in is meaningless, but it is his strident atheism that sticks out. "The Pope Song", for example, contains no less than 85 swear words. "It's a thing of great beauty because it's a challenge, a desecration of something people hold sacred. I want to make people realise that being angry about being mean about the Pope is completely inappropriate in the context of talking about child abuse." Nevertheless, on his US tour last month he found himself piano-less in Dallas when the hire company cancelled his contract, calling him a "God-hater" and a "demon". Such clashes have seen him called the Richard Dawkins of comedy.
Fame doesn't bother Minchin, but he worries how it might affect the two children, Violet and Caspar, he has with his wife of 10 years, Sarah. They live in Crouch End, north London – "in a house I bought with my songs".
"I've had to come to terms with the idea that I might have a lot of money and that comes with a whole lot of ethical problems," he says. "I'm not scared of the idea that my work is a business and that my image is a brand. You'll never see me advertising a product. Pepsi just asked me and it's so easy to say no. Do I want to sell Pepsi? No, I want to sell my ideas."
'Matilda The Musical', Cambridge Theatre, London WC2 (0844 412 4652; www.matildathemusical.com) to 17 February; Tim Minchin and the Heritage Orchestra is out now on DVD (Universal Pictures, £19.99)