OK, so I have to admit, it was a little disconcerting to see Sacha Baron Cohen without his Borat moustache. When the lanky comedian showed up for his first newspaper interview as himself since the inception of Borat-mania last autumn, Baron Cohen looked a little smaller than life, especially compared with the outsize character who caused such a sensation in Borat, the hit that managed to be something for all people, whether you laughed at Borat's outlandish behavior - or the people who indulged it.

Sipping hot lemon tea at a coffee shop in Santa Monica, California, Baron Cohen had the air of a man who had shed a layer of skin that had been worn to a frazzle. With a thatch of unruly black hair and a three-day beard, wearing a rumpled corduroy jacket, the 35-year-old comic could pass for a young film professor at UCLA without attracting a second glance. His thick Kazakh accent was gone, replaced by a sober British purr.

Most comics drop the act when the movie finishes. But for months last autumn, wherever he went, Cohen arrived in full Borat drag, taking the Toronto Film Festival by storm, holding a news conference outside the Kazakh embassy in Washington and, while accepting a magazine award, praising Mel Gibson, saying, "It is you, not me, who should receive this GQ award for anti-Jew warrior of the year."

It was brilliant marketing, with news organisations happily turning their reporters into straight men for a series of madcap interviews. In a way, Baron Cohen's still at it. His publicist first called with the idea of Baron Cohen doing an interview - as himself - the day after he scored a Golden Globe nomination (he duly won the award for best actor in a musical or comedy on Monday). Coincidence? I think not.

Still, the burden of being Borat took its toll, especially during months of filming when, to keep up the charade, he was Borat from dawn to dusk.

"It was exhausting," he recalls, slumped in the booth, fighting off a nagging cold. "I had to be that way all day and all night, because even if the tiniest detail had gone awry, it could've made them suspicious. I mean, even if I went to the bathroom, I had to make sure I went to the bathroom as Borat." He allows a tiny sliver of a smile. "There would definitely be potpourri in the toilet, so you'd know Borat had been there."

In an era when entertainers have to apologise for any sort of intemperate remark, Cohen cleverly created a comic character that provided him free passage for all sorts of outrageous behaviour, be it lewd remarks about women, mocking of worshippers at a Pentecostal church or a visit to a gun shop where he asked the proprietor, "What is best gun to defend against Jew?"

Having perfected this sly shtick in television on Da Ali G Show, and its predecessor, The Eleven O'Clock Show, where he posed as a gold-chain-encrusted hip-hop dunce, torturing a variety of government officials with wildly inappropriate questions, Cohen has become a master provocateur. Borat seems uniquely suited to our time, even if the character has deep comedy roots. If you wanted to see a nice Jewish boy assume an ethnic identity as a way to passing himself off as a cartoonish insult artist and womaniser, you could watch Chico Marx in any Marx Brothers comedy.

And though nothing can top the incomparable Ali G Show sketch in which Borat gets a crowd to sing along to Throw the Jew Down the Well in a redneck bar, it owes a debt to the work of Randy Newman, who would gleefully encourage similar singalongs to Short People ("They got little hands, little eyes, they walk around telling great big lies") as if he were endorsing the song's unsettling sentiment. Cohen's breakthrough is that he presents his comedy in a realistic setting - with recognisable people, people who might live next door, as foils. It gives his bits a barbed authenticity that is often as troubling as it is funny.

"Throw the Jew Down the Well was only interesting because the people in the bar started to sing along," he explains. Some of the people Baron Cohen and his director, Larry Charles, filmed say their actions were taken out of context, a charge Cohen vehemently denies. "If you saw all of our footage with the gun shop owner, for example, we had a whole conversation about the right gun to use to shoot a Jew's horns off his head."

None the less, a number of people in the movie have complained or filed suit, claiming they were hoodwinked. Cohen isn't exactly sympathetic. "This wasn't Candid Camera," he says. "There were two large cameras in the room. I don't buy the argument that, 'Oh, I wouldn't have acted so racist or anti-semitic if I'd known this film was being shown in America.' That's no excuse."

Borat was produced by Jay Roach (the director of "Meet the Parents"), who likens Cohen's comedy technique to the work of a gifted magician. "You know it's a contrivance and that you're going to be fooled, but then there's this extra layer of reality that takes you past the amazement factor and to a place where you're not even sure it's a trick any more," he explains. "Sacha's a real student of comedy, so he's incredibly thorough."

Born into a middle-class family in London, Cohen had early dreams of being a basketball player or a breakdancer. He spent a year on a kibbutz as a teenager and was a member of Habonim, a Socialist-Zionist youth movement that, he jokes, "basically meant that we shared our sweets". He was ambivalent about becoming a performer. "I think I was embarrassed to admit to my friends or myself that I wanted to be a comic - it was sort of like admitting you wanted to be a model."

At Cambridge he read history, spending a summer in the US researching a dissertation on the prominent role Jews played in the American civil-rights movement titled The Black-Jewish Allies: A Case of Mistaking Identity. As the title suggests, he was already fascinated by the notion that irony and identity play a big role in cultural differences.

"I was writing this at the time of the Crown Heights riots when the Jewish community was obsessed with black anti-semitism," he explains. "And I argued that this obsession came out of Jews feeling betrayed by their old blood brothers from the civil-rights movement. But while it was perceived in the Jewish community that Jews were disproportionately involved in civil rights, my conclusion was black Americans didn't see Jews as being more involved than any white Americans.

"The Jewish kids were all there in the South, but because they were there as part of church organisations like the [Southern Christian Leadership Council], they weren't seen as Jews but as white liberals. So there was this deep irony that the Jewish establishment took martyrs like Andrew Goodman and Michael Schwerner" - two civil-rights workers from New York who went to Mississippi to register black voters and were killed by the Ku Klux Klan - "and used them as symbols of a Jewish-black alliance when, in fact, they didn't really see themselves as Jews at all." Cohen pauses, drolly adding: "The dissertation is a lot funnier than I depicted it."

Not long after graduating from Cambridge, Cohen found himself drawn to the hip-hop scene in London, where he became a fan of the DJ Tim Westwood. "I'm sure he helped inspire Ali G," he says. "I'd thought he was black, because he sounded like a New York gangsta, but he was actually a tall, skinny, white guy who was the son of a bishop."

Soon Cohen was creating Ali G-style sketches for TV - he first appeared on the Paramout Comedy Channel, with two-minute segments in the guise of Bruno, the gay, Austrian fashion reporter. Borat followed soon after. A stickler for authenticity, during filming for the movie he never washed his grey Borat suit and never wore deodorant: "The smell is an added thing for people to believe that I'm from a country where hygiene wasn't a necessity."

By his count, people called police 37 times during filming, not counting the time Secret Service men showed up when he was outside the White House "figuring we must be al-Qaida, since why else would two guys be driving around the White House in an ice cream truck".

His closest escape came in Louisiana, when a woman whose family had once been plantation owners was insulted by a question he asked her and instructed her husband to call the police.

"We had 30 seconds to make our getaway in an ice cream truck whose top speed was 50mph," he says. I asked Cohen what he said to insult her. He furrows his brow for a moment. "I'm not sure," he finally responds. "But I think I might have been trying to sell her some Kazakhi slaves."

· Borat: Cultural Learnings of America For Make Benefit Glorious Nation of Kazakhstan is released on DVD on March 5