The video has been watched close to 10m times on YouTube alone. It starts with a cute brunette in a slim-fitting purple top and slacker combat trousers, standing in a cool, white designer hotel room and picking out a gentle tune on an acoustic guitar. “Hey, Jimmy,” she smiles. “It’s me. I’ve been on the road so long, I don’t even know what city I’m in. I’ve been thinking about you a lot, and I’ve been needing to tell you something. We’ve been together for over five years, and I still haven’t told you. And it’s just not right, so here it goes.”

Power chords cut in, she grins with suppressed excitement and wails: “I’m f***ing Matt Damon!” Cut to the actual Matt Damon sitting on the sofa in the suite, nodding with quiet approval. “She’s f***ing Matt Damon,” he agrees, following up with a lascivious growl: “On the bed, on the floor, on a towel by the door, in the tub, in the car, up against the minibar . . .”

The clip spins out into a parody of a hip-hop video, with Damon standing in baggy orange overalls in front of a pack of homies, giving it a knock-knock-style call and response: “Who’s that knocking on my door?”, “I’m fuh . . .”, “I’m fuh . . . who?” Even if you haven’t yet been prodded into viewing this Emmy-nominated skit online, you can almost certainly guess the next phrase.

The only questions are: who is the pretty, impish singer? And how on earth did she persuade the Oscar-winning A-list mega-star Damon to take part in this glorious tomfoolery?

She is Sarah Silverman and, as far as this side of the pond is concerned, she’s American comedy’s best-kept secret - although that’s about to end, as her breakthrough 2005 movie, Jesus Is Magic, finally opens here on Friday. Damon took part because of a long-running Tinseltown in-joke. Until recently, Silverman went out with a late-night talk-show host called Jimmy Kimmel, who hosts ABC’s rival to The Late Show with David Letterman.

“Jimmy ended each show by saying, ‘Sorry, Matt Damon, we ran out of time,’ ” Silverman explains. “He started saying that out of frustration when his show was fairly new, because they couldn’t book the big-name guests - so it was a funny notion that Matt Damon, a huge star, would be bumped. Matt thought that was funny, and would do bits for the show. He agreed to do this one and surprised Jimmy with it on his fifth anniversary show.”

Kimmel’s response came on Oscar night, with the video I’m F***ing Ben Affleck, a We Are the World-style anthem that featured, among others, Affleck, Brad Pitt, Don Cheadle, Cameron Diaz, Macy Gray, Robin Williams, Meat Loaf and Harrison Ford. Astonishingly, the happily married Damon played up to the gag in on-air interviews for months afterwards, telling an ITN reporter that the first video had been a private thing between him and Silverman, and that he was surprised at Kimmel’s petty response - adding a mischievous justification that, after all, he had been boffing Kimmel’s girlfriend for quite some time.

The episode briefly made Hollywood look like a fun place to work, boosted Damon’s reputation by proving him to be an uber-star who could mock himself, and levered Kimmel and Silverman into the category of America’s filthiest sweethearts. Sadly, they split earlier this month. Our interview took place three days after the break-up had been confirmed by - say it ain’t so - the couple’s publicists. Perhaps unsurprisingly, Silverman asked for the interview to be conducted by e-mail and said she would prefer not to answer personal questions.

The comic has a history of depression - although you wouldn’t guess from skilfully phrased, deeply politically incorrect one-liners such as “I was raped by a doctor. Which, for a Jewish girl, is so bittersweet”. At 13, she was on a school camping trip when a dark cloud of despair fell upon her - she missed three months of school and spent the next three years in a deep depression. Her parents took her to a psychiatrist, who gave her Xanax and told her to come back the following week. When she did, he had hanged himself. She had to sit in the waiting room for an hour until her mum picked her up. So it seems fair to cut her some slack on the e-mail thing.

Her answers, inevitably, were downbeat. Trying to ingratiate myself, I explained I’d prefer to use a quote to describe her sitcom, The Sarah Silverman Program, because I’m a cheap journalist. “I’m not sure,” she replied. “You’d think by now I’d have some kind of nutshell spiel, but I don’t. It’s about me, and my sister, who supports me financially because I’m an apathetic idiot. I have two big, burly, gay neighbours, and my sister has a boyfriend who is a big, square cop. I think I can accurately describe my character as an arrogant ignorant. And usually there’s a song somewhere. That was a shitty, circumlocutive and yet surprisingly uninformative description, wasn’t it? I’m sorry. If you weren’t so cheap . . .”

A comic since she dropped out of New York University at 19, Silverman, now 37, worked bit-parts in movies such as There’s Something About Mary and School of Rock until she let loose her on-stage persona on the talk-show Late Night with Conan O’Brien, telling a joke that hinged on the word “chink”. In the subsequent row, her profile soared.

Shortly after, she got into trouble for her contribution to The Aristocrats, a documentary with 100 comedians telling the same filthy joke. She implied, through an emotional soliloquy, that she had been sexually abused by a Jimmy Tarbuck-style showbiz veteran called Joe Franklin. He threatened to sue, but Slate magazine praised Silverman as “the only comic in the film who met the challenge of the joke: she pushed it too far”.

“I don’t care if you think I’m racist,” she says in her act. “I just want you to think I’m thin.” Does she mind when people miss the gag? “Nah. It’s okay. Comedy – like pretty much everything – is subjective. How can you argue someone’s earnest opinion? As a comic, you have to accept you won’t be for everyone.” She has several jokes with the word “nigger” in them, and when there are black people in the audience, she gets scared, but she feels that editing the words out in that situation would actually make her racist.

So, do gags like the following – “I love how Palestinians and Jews hate each other.

It’s so cute. Honestly, what’s the difference? They’re brown. They have an odour. It’s like sweet potatoes hating yams” – mean she courts controversy? “I try not to deconstruct myself. It can be really identity-crisis-inducing, you know? It’s like if someone says to you, ‘I love when you do that thing you do with your mouth when you laugh’ – then you can never do that thing again, because you’re too conscious of it. Does that make sense?”

Race plays a big part in her routines – she thinks it’s one of the key issues in America today – and she supports Barack Obama wholeheartedly, describing him as “a real leader you can get behind and be excited for”. But she will take a pop at celebrity culture, too: she gleefully tore into Paris Hilton at the MTV Awards, shortly before the hotel heiress checked into prison. Silverman worried that the phallic prison bars might hurt her teeth. Hilton herself was in the audience, and the camera lingered on her frozen face before focusing on Jack Nicholson, roaring with delight a few rows down.

Silverman is hoping to perform a few dates in the UK in the autumn, although she jokes that she’s been told to take her own lunch. It will be her first proper live performance here, although she delivered material at Amnesty International’s 2006 Secret Policeman’s Ball in London, when she was surprised to find that she liked Russell Brand’s material.

In the meantime, Jesus Is Magic is the best chance to catch her unexpected and overpowering comic scent. Poignantly, the film’s title comes from a gag about her relationship with Kimmel: “If my boyfriend, who’s a Catholic, and I ever have a kid, we’ll just be honest with it. We’ll say that Mommy is one of God’s chosen people, and Daddy believes that Jesus is magic.”

Given the chance to plug the movie, she decides to skirt that one. “This is a stand-up movie, but with scenes and songs,” she explains. “I got the idea for my TV show from loving said scenes and songs. In that way, the movie was almost the pilot for my show. Only very different. How’s that?”

The Sarah Silverman Program is on the Paramount Comedy Channel; the Matt Damon video is still on YouTube; Sarah Silverman: Jesus Is Magic opens on Friday in London and Edinburgh. For further information, visit www.myspace.com/comedybox

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