Carrie Fisher is having an up day. She talks in amphetamine bursts, flips her pen, bounces in her seat, hugs the footstool, paces her Savoy hotel suite. If this were a down day, things might not be so fun — but what else can you expect from the self-appointed ambassador of manic depressives? Fisher’s public path has taken her from Princess Leia to Hollywood’s ministering mental-illness angel. We know act one of her life: the daughter of dysfunctional Tinseltown stars (Debbie Reynolds and Eddie Fisher) stumbles into fame as the strangely coiffed eye-candy in Star Wars and then descends into drink and drugs mayhem. It was the story of her celebrated book Postcards From The Edge.

Now in her new comic novel, The Best Awful (Simon & Schuster, £14.99), the heroine stops taking her antipsychotic medication, gorges on bad men and drugs, then spins into mental hospital via a psychotic breakdown. Comic novel? Well, biography disguised by a Groucho Marx moustache. Names changed to protect the guilty. “A lot of people in Hollywood are manic depressive and don’t talk about it,” she says in her gravel-glam tones. “I tried to illustrate the condition comedically but give you an idea of what it is like. I think there’s a point to that rather than it just being some dittery confessional.”

Thank the Lord. We hardly need yet another “I went through drink/drugs/sex/madness hell but I’m over it now and here’s my shiny new life and trophy partner”. Instead Fisher has created a searing comedy of derangement by baring her psychic wounds. Manic depression (or bipolar disorder, its Sunday-best name) is no joke — or isn’t if you are in the pits of its despair. On the other hand, manic highs can make the world a bright, intensely exciting place. Huge ups, massive downs: it’s the perfect Hollywood syndrome.

It says much for Fisher’s no-shit demeanour that the glitterati didn’t abandon her once that dread spectre was diagnosed — a mental illness. “Most of my friends have been positive, though some are at the concern/pity stage,” she says. “They look at you with their head on one side and say, ‘How are you?’ There is still a certain amount of concern in the back of people’s heads that I might blow up again.”

And she has a new role: nursemaid to Tinseltown’s mental meltdowns. “I have become a resource if someone’s in trouble. Maybe it is more fun to have a celebrity rescue than to go to a physician,” she says, her entire teensy body jiggling. “The things people ask for help with have steadily got more Gothic. You don’t want to say, ‘I don’t know where you can get help for that’. I had to learn. I started calling California University for advice.” She waves an academic hardback: Suicide in the Entertainment Industry. “Great, huh? Tracey Ullman gave it to me for Christmas. People assumed I knew it all because I had gone to mental hospital and through rehab. Part of me wants to say, ‘How the f*** would I know?’ but you can’t.”

What Fisher knows, and what sets her book apart from the standard celeb-crash genre, is the carnival of psychotic terror that comes with total breakdown. She takes you along for the ride. It reminded me why I stopped taking acid.

“Mania starts off fun, not sleeping for days, keeping company with your brain, which has become a wonderful toy computer showing 24 channels of TV all about you. That goes horribly wrong after a while,” she says.

But she has a survivor’s pride — you can almost see her wearing a superhero outfit: ManicWoman. “I don’t think of myself as having a mental illness, but it’s a matter of definitions. There is this awful medical term, bipolar, which must have been thought up in a lab. The words manic and depressive have a grandeur. Coleridge was manic depressive. Annoying gits are bipolar.

“When I was in hospital there were two gangs: the dually diagnosed — people like me who had drug and drink problems as well as manic depression — and those who were ‘just bipolar’. We were the cool crazies. I went to a just bipolar evening a while back. It was deadly.

“I am spending more time now than ever helping bipolar patients and organisations. There is a place in LA that treats children with manic depression, which I help with. They said they wanted help getting a drama teacher. That’s the last thing they need!” Now the book is out, Fisher is working on two sets of scripts for television series as well as raising her young daughter. “The treatment that sorts me is a combination of medication and the talking cure. I have worked out a way of living, a regimented lifestyle — or a life, there isn’t much style. If I was playing truant it would show up. I exercise in the morning, work in the afternoon and go to the 12-step thing. I have my own kind of normal now.”