Tony Slattery once had the dubious distinction of being voted BBC2's most Boringly Ubiquitous Personality Of The Year. He defiantly relished the title, particularly as he was the only nominee for it. When one critic called him the most dysfunctional personality of the 20th century, Slattery enjoyed that too. Slattery's offence, in both cases, was to be seen on TV with almost promiscuous regularity. Whether as talk-show guest, game-show host, film-show anchorman or leading player in the improvised comedy boom— he is best known for Whose Line Is It Anyway?— he was just a telly tart who couldn't say no. Always cocky, self-confident, cheerful, never lost for a slick answer, even, perhaps, smug. And then, three years ago, with critical jeers ringing in his ears, but no sign of a let-up in job offers, Tony Slattery disappeared from view. "I went mad," he explains. "I was barking. Absolutely bonkers. I believed the rest of the world was against me, and I didn't blame them because I hated myself, too. I found myself frantically rifling through rubbish bags on skips, searching for evidence that people were betraying me. I'd lay flat and hide under parked cars, convinced that someone was after me. At home, I'd lock myself in the laundry room, terrified that there were other people in the flat trying to kill me. I believed that I was being eaten from the inside by parasites."

It has been a long haul back to normality— or what passes for it in showbusiness— but Slattery is now ready to get back on to the merry-go-round. He stars with James Purefoy, Susan Vidler and Leslie Grantham in a British film, The Wedding Tackle, which opens August 11. He plays a sexually frustrated cartoonist who is determined to disrupt the nuptials of his best friend (Purefoy), and his ex-girlfriend (Vidler), for whom he still carries a torch. "I was determined not to go back to the old days of taking on everything, so when I was offered this part in what the producer called a romantic comedy, my heart sank. Those two words strike terror into me. Then I read the script and saw that it's a comedy that's anything but romantic. So I jumped at it." He felt sufficiently encouraged by this to take his first tentative steps back into live performance two months ago. London's Comedy Store asked him to turn up one night and improvise. "I was shaking like a leaf. I stood there, not knowing what I was going to do or say, and I wanted to throw up. Then there was applause and I realised that it was for me. When I walked out there, nobody threw anything at me. I thought: 'This is going well so far!' I did my stuff and when it was over I realised the whole evening had been a turning point."

He is now choreographing a show for the Edinburgh Festival. "I'm being more choosy, doing different things to the game shows that once filled my working life. I feel better, mentally and physically, then I did even before the demons crawled out of the cupboard." The bad times—as he now refers to that chunk of his life—began while he was starring in Tim Firth's play, Neville's Island, in London's West End. "I had to re-enact a man's mental and emotional disintegration. It was hard work and it required a lot more concentration than any of those TV game shows, which were dead easy. When I read about other actors talking about how their work takes it out of them, it makes me sick. Acting's just a job—it's not brain surgery. Anyway, the show was a success, which made me very proud, and I suppose it went to my head a little bit. But it left me exhausted, and then came a bad bout of depression, to which my family is genetically prone. I could have sought help, or medication, but I didn't. Instead, I went the other way and tried to self-medicate. I prescribed myself generous amounts of cocaine, amphetamines and alcohol. Sometimes I'd get through two bottles of vodka in a couple of hours— I didn't even like the taste. I found myself on a roller coaster of madness."

He would fly into rages for the most trivial reasons. Much of his anger and bitterness would be directed at his friends who became alarmed at the dramatic change in his personality. "I would sleep for four or five days at a time, the sleep of a truly exhausted man. But when I woke up, instead of feeling refreshed, I would still be tired. Then, I'd snap out of my apathy and spend days engaged in manic, pointless activity—talking non-stop and busy, busy, busy all the time. I would spend days, literally days, rooted to one spot in my flat, staring at a spot on the wall. For a year I never answered the phone, didn't even open a letter— all the posts piled up inside the front door because I couldn't bring myself to handle anything. Whenever the bailiffs called to break down the door and take my possessions away because I hadn't paid my bills I'd just write them a cheque and they'd go away happy. Luckily, I'd earned enough from the good times to keep myself afloat financially during the desperate period. And along with the bills, I was ignoring offers of work— which never stopped— and letters from worried friends.'

Slattery had moved into a second-floor warehouse flat in Wapping, overlooking the Thames with a view of Tower Bridge. He had very few possessions, but one day, Slattery snapped and decided to clear the flat completely. "I hurled everything out of the window. The phone, the fax, TV, the stereo, they all went, and I was just struggling to push the fridge out when the river police arrived. They called up to me: 'Tony, will you please stop throwing things in the water. Take the fridge back into the room, Tony. Tony, not the fridge! Oh hell, too late.' There was nothing in it, anyway, except for empty vodka bottles."

Curiously, once a week Slattery would somehow pull himself together and visit his Irish-born parents, Margaret and Michael, putting on a show of complete normality. He is the youngest of their five children—his older brothers, Stephen, Michael and Christopher are triplets and he has a sister, the oldest child, Marlene. Margaret was a home-help, while his father, who died last year, worked at the Heinz factory not far from their home on the grim Stonebridge Estate in Willesden, north London. "I'd force myself to walk up their garden path, trying to pretend everything was fine, and what a performance that was. Of course, they knew what was happening because I was behaving badly towards everyone and the cracks were clearly showing." It must have been agonising for the Slatterys to watch their beloved youngest child destroying himself. They were fiercely proud when he went from Gunnersbury Boys' Grammar School to read Spanish and French at Cambridge, and for a time Tony set his sights on becoming an academic. Then he became friends with Stephen Fry, who encouraged him to join the university's famed Footlights revue. 'Dear boy, you simply must come and have some fun,' he told the overawed Slattery. Hugh Laurie and Emma Thompson were leading lights, and Slattery found himself writing sketches for them. He remembers 'lots of late nights with kebabs and bad red wine, trying to work out what was funny". In his second year, they were all signed up by a London agent, who had arrived in Cambridge wearing a camel-hair coat, smoking a fat cigar and driving a powder-blue Rolls Royce. "I remember thinking: 'This is the big time'," says Slattery.

The team were briefly reunited in the early Nineties in the movie Peter's Friends, directed by Kenneth Branagh, then married to actress Emma Thompson. There were cries of luvvie nepotism, but Slattery says it wasn't like that. "I never socialised with them, and I hadn't worked with Stephen for ten years. And I'd never met Branagh, so I had to audition for it, fair and square." Indeed , since Cambridge, Slattery had immersed himself in populist TV panel games and quizes while the others strove for higher intellectual planes.

Although Slattery was hoping to conceal his mental breakdown from his parents, his mother was finely tuned to him. "One night, I was driving home, and I'd had too much to drink. I swerved to avoid a cat, lost control and crashed into a car right outside Wapping Police Station. I was hanging upside down, held by my seatbelt, when a great shard of glass went into the passenger seat. It was a matter of just a few inches and I would have been speared right through the heart. The police report said this happened at 4:05am. Later that day, my mother called me to say she had been woken up at home by her dressing table mirror shattering during the night. It had happened, according to her bedside clock, at 4:05am." The turning point in his life came soon after. "I'd got to the bottom of the pit. The only other option left to me was to finish it, kill myself. I discounted that straight away, because it's not a brave act, it's selfish. So, the only way was up. I saw a psychiatrist, who told me: 'You're going into hospital right now. You need help, you're out of control.' From that moment on, my recovery began. I understand that my illness is unlikely to happen again, but if it does, I know who to turn to. Now I have got some, but not all, of my friends back and my family can look into my eyes and see I'm better. I realise the hurt I've caused them, but at least I'm not scary anymore."

Now, at 40, he looks younger and fitter than ever. He has never married, although he was engaged when he was at Cambridge (that came to an end when he found his fiancée in bed with another girl, and although he jokes that he briefly agonised over whether to stay and join it, he went). There has been little love interest since, and he dodges questions about his sexuality: "What I do is my business. I don't care if people call me gay, but the only person who knows the truth is me— and it might change from day to day." All he wants to do now, he says, is to get on with a life he put on hold three years ago. "Whatever normal is, I'm normal now. Until recently, I hated myself more than I could ever hate anyone else on earth. I was filled with self-disgust, self-loathing. Now I am at ease."

Recently he had an encounter with Christine Hamilton (wife of disgraced former MP Neil) for BBC Choice. She took him to Piccadilly Circus, stood him beneath the giant adverts for Coca-Cola and Sanyo, and pointed to the moving screen above. "You've got 15 words to put your own message up there," she told him. "Say whatever you like." For Slattery it was easy— even though the crowds in Piccadilly might have been puzzled. He wrote "Tony Slattery says sorry to all the people he's neglected to cherish." He smiles. "I'll never wipe the slate clean after all the terrible things I did during the bad times," he says, "but it's a start."