After a year as a psychology student at the University of California, Ruby Wax dropped out: "I was going to be a shrink but I thought if their stories got dull, I'd have to kill 'em." So Ruby ended up in television, still with an aversion to dull stories. Tapping into the inherent vulgarity of her victims with the unerring instinct of a water diviner, it was Ruby who persuaded the Duchess of York to let her rifle through her fridge and bedroom drawers, examined Zsa Zsa Gabor's neck for signs of face-lifts, and encouraged Imelda Marcos to warble Feelings to a piano accompaniment. In next week's edition of Ruby Wax Meets on BBC1, the show's eponymous star has the chutzpah to interrogate O J Simpson about his love life - after which, for a bit of fun, O J stabs her in the back with a banana. Some people can get away with murder.
 
Ruby is not everybody's cup of tea, but in a country both suspicious and patronising about noisy Americans, the motormouth from Chicago has undeniably carved her own niche in the popular culture. She sometimes gives the impression of not liking England very much for its own sake ("New York's too hard, too tough, and there are too many girls like me") but she can hardly complain that doors have not opened for her. The Princess of Wales became a confidante (hence the Fergie programme), enjoying lunchtime psychobabble conversations with the interviewer, who herself had spent years in Jungian therapy. Wax has never admitted as much, but it is claimed she gave Diana the line "There were three of us in this marriage", for her Panorama interview.
 
The family background of Ruby Wachs - as she was born in the leafy Chicago suburb of Evanston 45 years ago today - was as bizarre in its way as anything that engulfed the house of Windsor. Her father was an Austrian Jew who made bratwurst skins before fleeing the Nazis and making a fortune by transferring his epidermal expertise to hot dogs. Her mother was obsessively house-proud, covering her furniture in clear plastic against the depredations of her only child. For eight years dance lessons were compulsory, her mother standing guard so she could not abscond (a favourite Ruby ruse). "I remember her ripping the door open and screaming, 'Get your finger out of your nose and dance!' " says Wax. After attending an "experimental" school that almost rendered her illiterate by teaching her to spell phonetically, she ended up at finishing school in Switzerland. Two months later she climbed over the balcony and fled to London, where, penniless in the King's Road, she cabled her father for money. To teach her a lesson he wired her a dollar - and her mother came to collect her. The University of California at Berkeley followed, but after she quit psychology she headed back to London, hoping to train as an actor. Only the Royal Scottish Academy of Music and Drama in Glasgow would take her ("It was a dump"), but she liked the roughness of the city. "I felt I fitted in. English people had always made me feel like an outsider."
 
It was in Glasgow in 1976 that she entered into a marriage of convenience with a theatre administrator in order to stay in Britain. The pair divorced in 1980. Wax had found her way to the Crucible Theatre in Sheffield, and then - to her unconfined joy - to the Royal Shakespeare Company. She was not a great actor ("I only got in because I made Trevor Nunn laugh") but performed "wench parts" energetically. Sharing a stage one night with Judi Dench she realised she had better find a new career.
 
Encouraged by her theatrical friends she wrote a play, Desperately Yours, which was performed off-Broadway and led to an invitation to write sitcoms in Los Angeles. The move was a disaster. Her work was constantly rejected, and for nearly two years "I lost my mind". She married an Englishman named Trevor Walton, a match she says was another "convenience" arrangement (this time, on his part) but which friends say was a proper romance. In any event, it didn't last and she was soon back in London, writing comedy scripts for Not the Nine O'clock News and then collaborating (and appearing with) Dawn French, Jennifer Saunders and Tracey Ullman in Girls on Top.
 
Ed Bye, the director of Girls on Top, became her third husband. They both got drunk at an end-of-series party, kissed in a hotel corridor, and went to Bye's room. Wax takes up the story: "The first night we slept together, Dawn [French] was waiting up all night to see if we had sex. I had to report to her at 5am the next morning. He said, 'Are you telling Dawn about us?' I said, 'No.' Then I walked out the door and crawled, commando-style, down the hallway until I got to Dawn's room and we were hysterical."
 
Girlie hysterics in the powder room are an integral part of the Wax formula - her interviews with men are less successful, possibly because they find her disingenuous and strident - but the format of Ruby Wax Meets evolved almost accidentally from the early vehicles for her talent. One day her guest on The Full Wax, Tammy Faye Bakker, refused to fly over, so Ruby flew to her and did the interview at home.
 
She felt it went better without an audience, and it is certainly difficult to imagine the subsequent triumphs of 1996 - Fergie, Marcos, Gabor, not to mention Pamela Anderson demonstrating her favourite sex positions - taking place in a studio.
 
The appeal, which can be toe-curling, is essentially a conspiracy between Ruby and her audience: someone, somewhere, is going to get set up. "Gradually, they have seen I'm playing with my guests," says Wax.
 
"Imelda Marcos was the most outrageous game I've ever played and yet, when I left, she presented me with a lot of gifts. She wasn't angry, she was pleased." Despite locking the hapless Duchess of York out of her own house at the end of the interview that 15m people watched, she insists that malice or mockery was not intended: "If you saw it as an attack, it wasn't. I just didn't understand her Englishness." (The duchess, like Imelda, was full of thanks afterwards.)
 
One suspects that Wax understood perfectly well what was going on, but journalists who raise the matter of Fergie - or Wax's slightly mysterious relationship with Diana - tend to get snapped at. "You piss me off," she told one interviewer from The Independent, after switching off her tape recorder. She also reacts sharply to questions about her Jewishness: "Do not put in that I am Jewish. It's irrelevant. It means nothing to me." In her opinion the servile American press is wonderful, whereas the British press is the pits. Ruby can dish it out, but doesn't much like the sound of incoming artillery.
 
This is possibly because she is not sure whose side she is really on. The Marcos interview was secured partly because Wax had appeared as a Hello! cover story and was perceived by the great shoe collector to be a star in her own right. Just to be on the safe side, Ruby borrowed £100,000 of Theo Fennell jewellery to wear in Manila.
 
There is also the problem of her burning wish to be taken seriously as a documentary maker. She has made some interesting, idiosyncratic films - including Miami Memoirs, about her strange parents - but she clearly feels stereotyped: "Just because I don't do serious stuff doesn't mean I don't know how to make a documentary, and I'd like some day for people to say I'm a good documentary maker instead of some stoopid comedienne."
 
In middle age the ambition is still quite naked. In an interview with John Goodman, the Roseanne star, (who plainly couldn't stand her), Goodman remarked how good Terry Wogan was. "He's over," Wax shot back. "I'm his replacement." She has also managed to mix her career with a happy family life (three children) about which she is refreshingly candid. Asked whether she misses the children when she goes away, she replies: "I do at first, but I soon forget about them." And if she feels like losing her temper with the kids "I walk out and let the nanny deal with it". One ambition will remain unfulfilled: "I'd like to be a political interviewer but I don't know enough. Somebody who's a celebrity is just as dumb as I am, so I've never been scared of them." Perhaps the way forward is for Ms Wax to make a documentary about artistes who operate in a similar genre to herself: Dame Edna meets Mrs Merton and Dennis Pennis. Or would that be in bad taste?