When Carrie Fisher goes missing right before an interview, you have good reason to worry. This, after all, is the woman who stopped taking her lithium during a holiday in Australia, had a psychotic episode, and took an impulsive trip to China because it was only six inches away on the map.

Her press officer soothingly insists that, on this occasion, Fisher is simply out getting some fresh air, psyching herself up to talk to me, and in fact – look – here she comes now, through the revolving door and into the foyer of the Savoy Hotel. “Sorry,” she says in her familiar husky drawl. “I was trying to buy a Valentine’s gift for my daughter, Billie.”

What did you look at, I ask.

“A pair of vibrating breasts.”

Anything else?

“A card with George Bush dressed as Osama bin Laden.”

And?

“An inflatable sheep.”

By this time we are in the room where Fisher is meeting the press while in London. She throws her bag on the floor and herself on the couch. She’s really small. Her hair is a blondish, brownish bob. A swag-necked top swoops off her right shoulder. Her black skirt is short and spangly. As she talks, gesturing expressively, bangles do the hula round her wrists.

We are here to talk about her fourth novel The Best Awful; the sequel to her justly celebrated Postcards From The Edge, it is a typical Carrie Fisher book in that it is an unashamed fictionalisation of the life of Carrie Fisher. She has always been a neurotic who gossips about her own neuroses – Hedda Gabler and Hedda Hopper rolled into one.

Postcards dealt with her time in rehab after a drug overdose, Surrender The Pink with her short-lived 1983 marriage to Paul Simon, and Delusions Of Grandma with her first faltering steps into motherhood. The Best Awful chronicles the period during the mid-Nineties when Fisher’s partner Bryan Lourd, now a Hollywood super-agent, left her and their young daughter for a man. The novel also covers the subsequent saga in 1997 when Fisher suffered a complete breakdown, was committed to a mental hospital and diagnosed as manic depressive. She spent several weeks in the hospital and was an outpatient for a further five months. She now takes a galaxy of pills to stabilise her moods.

Despite such trauma, the book is a hoot. I tell her that it reminds me of Alan Alda’s line in Woody Allen’s Crimes And Misdemeanours about comedy equalling tragedy plus time. “Absolutely,” she says. “It took me a long time to get around to using this material because it wasn’t funny for a while. But the longer I waited, the funnier it got.”

Suzanne Vale is Fisher’s alter-ego in Postcards and The Best Awful. Like Fisher, Vale is the child of showbiz royalty, a sometime actress growing up in Hollywood disaffected and insecure, prone to drug use and introspection. I ask Fisher whether by turning her own experiences into works of fiction and giving them to Vale, she is disassociating herself from her own past, saying, “These things happened to her, not me.”

She likes this. “That’s great. That’s hilarious. That’s brilliant.”

Fisher is 47 and has been in therapy on and off since she was 15; there is little that a journalist can ask her that she hasn’t already asked herself, but she regards interviews as “eavesdropping on myself”, constantly hoping to be surprised by what comes out of her mouth, so there is always the possibility that a line of enquiry will throw up something new. In this case she grabs my question and runs with it. “If my life wasn’t funny, it would just be true, and that’s unacceptable. In fact, it’s like being in 12-Step. You get up and you say ‘I’m Carrie and I’m …’ whatever it is that you are. At that moment, you’re saying something that is true. You’re unburdening yourself. Well, this book is an elaborate version of that.”

As soon as she puts her experience into language, she says, it becomes less about her. “Because it’s observed, it’s encased, it’s outside of me. In fact, it’s true that you’re only as sick as your secrets. I’m no longer run by it, I now own this thing. It’s material.”

And because you have turned it into a book, a creative act, it becomes something you can be proud of rather than ashamed by? “Yeah. If I can write about it and be able to be insightful about it, then I’m no longer just the asshole who did it.”

Reading and writing have always been a comfort zone, a safe space for her. “Books were my first drug,” she says. “It was like living in a forest of words; you could hang out and you knew it was going to be okay.”

This was in her early life. In 1958, when Carrie was two years old, her father, the crooner Eddie Fisher, left her mother, Debbie Reynolds, for Elizabeth Taylor; he subsequently wrote a candid memoir called Been There, Done That, which laid out his sexploits in intimate detail. Since then, her parents have been at war, and she grew up in the trenches. Books and black and white comedies provided a refuge. She loved that they had happy endings. They were “something reliable you could fix on”; Cary Grant always got the girl, Laurel and Hardy always remained friends despite all those fine messes.

Fuelled by such wisecracking fare, Fisher discovered that she too had enormous verbal acuity. Her mother and father were these fading movie stars, but who was she? Maybe she was a smart person, a wit, a reader of books. That idea of herself is the keystone around which her whole identity is arranged. Back in 1977 when she was promoting the first Star Wars film, in which she played Princess Leia, she considered it important that she sound smarter in interviews than Harrison Ford, and he was quoting Kierkegaard, so it was tough.

The Best Awful is a showcase for her compulsive wordplay. No phrase is left unpunned. Even she admits that it can get a bit annoying, but she can’t help herself. Give her a project, such as the Oscar speech she is currently working on for Sofia Coppola, and she will cheerfully lock herself away for hours with a dictionary and thesaurus. “Homophiliac,” she says, “I made up that word – cut me and a lot of gay men come out. It’s just constant. My brain does it without my permission. It’s happening up there.” She taps her temple. “It’s like having a tenant. I’m like ‘Shut up! It’s three o’ clock in the morning! Can I sleep?'”

Most of her self-esteem is based on this idea she is clever and insightful. That being the case, I say, how did you feel about the fact you hadn’t realised Bryan Lourd was gay? “That was a huge thing. As a smart person, ultimately who I had to forgive was myself. Because I was the one that didn’t see. It was my choice. It’s not like he was this master of disguise, and it’s not like he became gay. But, y’know, look, I’m not the queen of great judgement.”

But nobody expects the father of their child to come out to them, right? “Y’know, it was funny, I was on the Today Show and Katie Couric said to me, ‘But I mean, ultimately, isn’t it kind of easier being left for a man?’ She meant that it wasn’t personal, that he left the whole sex, not just me. Like in The Godfather.” She adopts a husky mafioso voice. “‘Tell Michael it was just business, it wasn’t personal.’It’s very complicated. First it’s a betrayal, like any betrayal. That’s how it feels. Then the next thing is, ‘What an asshole I was!’ And that’s compounded in me by I’m the person who can see things.”

She didn’t find out straight away that Lourd was gay. She thought he was leaving because he hated her. Was it then a relief to discover the truth? “No way. Nothing was a relief at that point. What’s a relief is that now, a couple of years after the fact, we’ve made peace with it, and I’ve made peace with myself. Prior to that, it was a torment. I felt like a failure. I felt like a shmuck. I felt like a hundred things.”

Fisher is proud that she and Lourd have now reached an understanding without recourse to a lawyer. “My mother and father were at war my whole life, and I never saw him and I’m not going to have that for my kid.” Now they “co-parent” 11-year-old Billie. It seems like a thoroughly modern arrangement. They go on holiday together like a regular family. One time the touring party was expanded by Lourd’s boyfriend and a friend of Fisher’s. “That was when we went to Disneyworld.”

This is complex stuff, and unsurprisingly Fisher has sought to declutter other areas of her life. “My life is really simple now,” she says. “I don’t have relationships so I don’t have to have fights.”

Is that true, I ask, or are you being glib? “Well, not fights. I don’t fight. I don’t have relationships so I don’t have to perceive myself as being a disappointment to someone.”

That’s quite a sad thing.”Isn’t it? I realise it’s assinine to talk like this when you are in your forties, but it stems from the Eddie thing.” She calls her parents by their Christian names. “I disappointed him therefore he didn’t come round. So when there’s a conflict in a relationship as an adult, it feels like that’s what’s happening. That doesn’t mean it’s true, and I realise that now, but that doesn’t mean I wanna live it.”

Fisher always claims to have inherited her mental illness from her father, who is now 73. How does he feel about being outed as a manic depressive? “You mean the guy who is outing everyone as being not that good a fuck and having a big ass?”

That’s the guy.

“Oh, that guy.”

It’s a serious allegation to make.

“Allegation,” she scoffs, leaning so far forward on the couch that her left knee almost brushes the carpet. “I’m always very clear that he doesn’t say he is. And I don’t think of it as an allegation, which is sort of why I do it.”

You think of it as a hard fact?

“He’s a very seductive, charming man who didn’t sleep a lot, who just was a crazy sexual being that did drugs all the time and never stopped shopping. He would bankrupt four times, marry, what, five? I don’t know. He’s taken a lot of the medications I take.”

She says that the best example of his mania was the time he came back from Hong Kong with “175 silk suits in every colour – puce, orange, green”. She was always very much like him even though he wasn’t really around. When he came by the house, she would encourage him to let guests see the collapsed veins on his arms caused by injecting speed. “Dad, show everyone your tracks!”

He sees his daughter every once in a while. She has decided to forgive him for the trouble he caused with his book, but these visits cause difficulties with her mother, who now lives next door to her in Los Angeles. “He created a situation by calling my mother a bad fuck and a possible lesbian, or whatever the hell he did, so that when I see him it’s a betrayal. You don’t wanna rush over to her house and say ‘I saw Eddie today!”‘

I ask if manic depression is common in show business. “More than you know,” she sings. If you are taking drugs and drinking a lot then your behaviour often seems manic depressive, she says, so it is often hard to diagnose among Hollywooders. This was certainly true in her own case, although she believes that she took drugs (her hero Cary Grant once called her up, as a favour to her mother, to warn her of the dangers of LSD) as a way of self-medicating, dealing with mood swings and heightened states of awareness that were only later given a name. She was first diagnosed when she was 23 but wouldn’t accept it. “I didn’t believe it because I had friends who really sincerely were bipolar, in my opinion. They were truly loony. I have one friend who is manic depressive; he went to Bethlehem to see if he’d be recognised. He thinks he’s Jesus.

“And another friend of mine, he would put on a Hassidic beard, turn down the TV, do Swedish subtitles, drink perfume and then buy $30,000 worth of furniture for the elevator man. So, if you tell me that’s what I am, then no. That’s what they are, not me. The thing in the hospital was a little bit of a convincer, but still there’s points when I think ‘I am not’. Sure.”

The “hospital thing”. She suffered one of those meltdowns where your personality turns to guacamole inside your skull. Gianni Versace had been shot dead, and as she watched the story unfold on CNN, she imagined she was Versace, his killer Andrew Cunanan and the police. Once committed, she stayed awake for six days and six nights, an almost Biblical period, hallucinating that a golden light was emanating from her head, an entirely Biblical image. She now takes her medication religiously and is only occasionally tempted to stop.

By the time we have finished talking about all of this, Fisher is off the couch and pouring a Diet Coke, a beverage which is now the focus of all her addictive tendencies. She once wrote in a magazine article that she often looks up the meanings of words in the dictionary when she wants to “orient” herself, so I ask – as a kind of addendum to the interview – what was the last word she looked up. “Oh, I looked up gay!” she says. “It’s a hobo word! If there’s a hobo riding the rails, an older man, and a younger hobo comes on the train, the older one will take care of him, and it’s understood that there might be some sexual exchange.”

Maybe, I offer, you should write about hobophiliacs. “Oh!” she yells. “You could do something with ‘hobophiliac’! I wonder what ‘hobophiliac’ is? I’m gonna work on this.”

She looks delighted. “This is gonna fuck me up all day!”