When I moved to London in 1990, I knew that, in order to fit in at school, I had to educate myself about the important British celebrities. While my classmates helped me with regards to the canon – Noel Edmonds, Phillip Schofield, Cilla Black – there was one I found all on my own. Tony Slattery quickly became a source of fascination to me. He was such a ubiquitous presence on television (endless quiz shows and commercials), in theatre (Me and My Girl, Neville’s Island, which got him an Olivier nomination) and film (The Crying Game, Carry on Columbus, Peter’s Friends) that Private Eye ran a cartoon of him in which his answer machine message was, “Yes, I’ll do it!” But, like most people, I discovered him on Whose Line Is It Anyway?, the endearingly low-fi Channel 4 improv show that ran from 1988 to 1999.

Pretty much everyone on that show was great – Josie Lawrence, Mike McShane, Ryan Stiles and, of course, Richard-Vranch-on-the-piano. But Slattery seemed to be in a different orbit: a gifted actor and strikingly handsome, he vibrated with creativity and a barely suppressed inner darkness. You could never be sure how his skits would go, but you knew they would have a jittery brilliance to them, with a leftfield lyrical twist or an emotional gut-punch. With his manic energy, he reminded me of Robin Williams, and it was clear that if he learned how to channel his talent there would be no stopping him. And even if he didn’t, well, he would still be exciting to watch. But I was wrong. What happened to Slattery was not exciting. It was sad.

Slattery suggests that we meet at the Guardian office, which is the first time a celebrity interviewee has offered to come to me. “Well, I didn’t want you coming to horrible Edgware,” he says, which is the corner of north London where he lives. His face suddenly crumples in anxiety: “But I haven’t done wrong by coming here, have I? Did I make a mistake?”

Slattery pretty much vanished from public life in the late 90s, and while 20 years will change anyone, he looks at least a decade older than his 59 years, and close to unrecognisable from his Whose Line days. Where once he was energetic and prickly, occasionally accused of grating self-satisfaction and gratuitous cruelty (he once said Jeremy Beadle should be “clubbed to death”), the man I meet today is like a lost, anxious teddy bear. Heavy-set and visibly nervous, he is still hyper-eloquent, with that familiar melodious voice, but the syllables sometimes stumble on his tongue. It is noon and there is a faint smell of alcohol about him, although he promises he hasn’t drunk anything today. “I made a special effort for you,” he says with a sweet smile. As we walk through the office, I notice that he is limping.

“I’ve got to get my leg sorted,” he says, rolling up his trousers. His leg is purpled with vivid rashes and lesions. “It’s some kind of cirrhosis,” he says, unconcernedly. Whatever Slattery took out of life when he tore through the 90s British entertainment scene, life has since reclaimed its debt tenfold.

A sharply dressed Slattery on Whose Line Is It Anyway? in the early 90s Photograph: Ronald Grant

The ostensible reason for us meeting today is that Slattery is reuniting with some of his old Whose Line colleagues for a show in Edinburgh this summer. “So people can come to that and say: ‘Fuck me, I thought he was dead,’” he says. One of those colleagues will be Richard-Vranch-on-the-piano, who, pleasingly, remains one of Slattery’s dearest friends, and one of the very few who has stuck by him. Almost all his other celebrity chums and hangers-on vanished “when the money dried up, which was saddening. Yes, very saddening,” he says, quietly.

I suspect the real reason he has agreed to talk is that he wants people to know he’s very much not dead and, hopefully, to attract the attention of an agent. “I haven’t had an agent for a while and I want to get back into the swing of things. I had a very happy time until I went slightly barmy,” he says.

What, in fact, happened was that in 1996, at the age of 36, he had a massive breakdown. After 13 years of nonstop work, fuelled towards the end by a daily diet of two bottles of vodka and 10g of cocaine, he collapsed, physically and mentally. He alternated between what he describes as “terrible isolationism and an almost comatose state, and then terrible agitation, constant pacing, sitting inside with thoughts whirling round and round”. Multiple hospitalisations followed – “all voluntary”, he emphasises. At one point, he locked himself in his riverside flat for six months and threw all his furniture into the Thames.

“The river police came by and said: ‘Tony Slattery, we like you on television, but please stop polluting the river,’” he says, doing a jolly imitation of a policeman. He often breaks into impressions during our time together – of Ken Dodd, Terry Wogan, his mother – and while they are all excellent, it feels as if he is doing them out of an exhausted sense of obligation to keep me entertained.

Media coverage of this part of Slattery’s life has tended to focus on the substance abuse, but there was another, then-yet-to-be-diagnosed problem. “The manic part of me was not because of the drugs and alcohol. I think it was there already. But the drugs and alcohol certainly ignited it,” he says. From the beginning of his career, when he would go on stage he felt like “a match carelessly tossed into a bunch of fireworks”.

He was eventually diagnosed as bipolar. Finally, he could make sense of the duality in which he still lives – “the mania, finding things too exciting, then the withdrawal, apathy and bleakness”.

I ask if he thinks the way he binged on drugs and alcohol in the 90s had less to do with substance addiction than with the mania of his mental illness. “There’s no question. Bipolarity often presents itself as something else, like a rash can present as lupus or Lyme disease,” he says.

Slattery in the ITV comedy Tiger Bastable, one of many shows he appeared in during his 90s heyday. Photograph: ITV/Rex/Shutterstock

And was his tendency to say yes to all jobs also part of his bipolar nature? “Yes, and also I was worried, because I wasn’t born into money, so I didn’t want to lose it. But I lost the plot and the money.”

Slattery survived this period, just, thanks to the steadfast loyalty of his long-term partner, the actor Mark Michael Hutchinson, whom he met while performing in Me and My Girl in the mid-80s. The two are still very much together, living in Edgware with their cat, and talking about Hutchinson prompts a juddering sob from Slattery.

“He’s kept with me when my behaviour has been so unreasonable and I can only think it’s unconditional love. He’s certainly not with me for my money – we don’t have any money. It’s the mystery of love. I’m sorry – it makes me very emotional,” he says, trying to pull himself together.

Until very recently, Slattery always refused to discuss his personal life or even confirm his sexuality. I ask if he was shielding his parents, both of whom are now dead.

“Exactly – it was honestly never anything to do with embarrassment. I just knew Mum and Dad would worry themselves to death that I might have Aids. I think Mum knew – she always referred to me as ‘my bachelor son’. But it was just not talked about.”

Slattery grew up on a council estate in north-west London, the youngest of five children and the son of working-class Irish immigrants. He was a quiet child and very close to his parents. He was also a gifted athlete, at one point representing England in under-15 judo, and an even more talented student, acing his A-levels to get into Cambridge and study modern and medieval languages. He insists he never felt out of place there: “I knew I was from a different background, but there was a spectrum of people there, from neo-Trotskyites making pipe bombs in a bedsit to people with 14 hyphens in their name who go out shooting rare animals. So you find your friends, and I went there to enjoy everything Cambridge could throw at me.”

Which he certainly did, joining the Footlights in one of its classic heydays, alongside Emma Thompson, Hugh Laurie and Stephen Fry. Previously, Slattery had dreamed of a career in academia, but this got him hooked on laughter and applause – “two of the most addictive substances known to mankind”. Nonetheless, when these same Footlights reunited to make the 1992 luvvie-tastic Peter’s Friends, also co-starring Imelda Staunton and Kenneth Branagh (who directed the film), Slattery was cast as the oikish outsider. “I wasn’t part of their class, you see. That’s the thing. And as Kenneth Branagh said at the time, this is a film about friendship among people of the same background,” Slattery says.

Slattery (third from right) with Kenneth Branagh, Emma Thompson, Stephen Fry, Hugh Laurie and others in the 1992 film Peter’s Friends. Photograph: Allstar/Cinetext/BBC

But Branagh comes from a working-class Northern Irish background, I say. “That’s right, but he … you know,” says Slattery, and he holds his collar over his face as though donning a mask. I laugh, but he looks down, guilty about teasing Branagh. “I still feel like I should tread carefully.”

Back in the 90s, articles about Slattery always described him as talented, but this was often undercut with a reference to him being “hard to warm to”. “Dark” and “angry” also came up a lot. “There was a lot of rage at that time,” he agrees. And yet he says he had a happy childhood, a happy teenage life, a happy university experience, was doing the thing he loved as an adult. Obviously, none of those things is a bulwark against mental illness, but where did the rage come from? Slattery makes a deep, shuddery sigh.

“I have a feeling that what might have been a contributing factor is something that happened when I was very young,” he says haltingly.

When he was a child?

“Um, yeah. Not to do with family. A priest. When I was about eight.”

We sit in silence for a few seconds. I ask if he ever told his parents and he is so overcome at the thought he can only shake his head.

“A psychiatrist once said to me: ‘Bear in mind that some things are so deeply buried there is nothing to be gained by an archeological dig. Keep it buried,’” he says.

I look at this broken man in front of me, now bent over as if crippled by the weight of a secret he has carried nearly all his life, and ask if he thinks that advice worked for him.

“I think so, because it would have been another bloody thing to deal with, along with the booze, the bipolarity, the overwork, the feeling of being let down by friends, my own bad behaviour. I think that’s enough of a cocktail to be getting on with. Some things are so horrible they serve no purpose to be relived,” he says.

I can feel both of us starting to recoil from the subject, him because he doesn’t want to think about it any more and me because I am terrified of pulling on the thread that will cause him to unravel entirely. So to move on, I suggest that maybe he has partly worked through his feelings about what happened to him, if he feels able to mention it now.

“Yes indeedy,” he says, but it is the one time in our conversation when it sounds as if he is speaking on autopilot instead of being emotionally truthful.

‘I’m getting better, but there’s still some way to go.’ Photograph: Graeme Robertson/The Guardian

Not all celebrities who disappear retire into gated-community comfort in Surrey and, contrary to the lie we are sold, fame is no cushion against falling between the cracks. Slattery is charming company – sweet, solicitous, his brain somehow still sharp despite his best efforts to blunt the thoughts that tormented him. He gave up the coke around the millennium when his beloved mother found some in his flat and he was mortified into abstinence. He couldn’t afford it now anyway. When I ask what his plans are this week, he says: “Buy some food, because we’ve run out. But we’re waiting for money to come in from jobs and that often takes a while. So just make it to the weekend.” It is very hard to not measure the distance between what is, what was and what should have been. He does still drink and, yes, he knows it would be better if he stopped completely, but he doesn’t think he has the strength to do that. I tell him I am worried that performing will make him drink more. “I’ve been quite strict with myself so far,” he says. “But there have been times when I’ve thought: ‘I can’t go on stage, I need that half bottle of vodka right now.’ I’m getting better, but there’s still some way to go.”

He is no longer under psychiatric supervision, and he stopped taking his last lot of medication two months ago. “There was a numbing effect and I thought: ‘I can’t live in this state where you can’t feel or see anything.’ I’d rather be exposed to the fire.”

I walk him back through the office to the front door, where he hugs me goodbye and apologises for “garbling a load of melodramatic nonsense”. I tell him he did no such thing. I hope an agent gives him a chance, because he deserves more than this, and I tell him so. He smiles and says he just hopes he makes it to his 60th. But that’s only in November, I say. “A lot can happen between now and then,” he replies, cheerfully. And then he walks away, back on to the streets, exposed to the fire.

• Bipolar UK can be contacted on 0333 323 3880 or email info@bipolaruk.org.