Before we meet Miss Fenella Fielding, I should remind you of her most famous work.

Not only was she a star of comedy stage shows in the 1960s and a regular on Morecambe and Wise in the 70s, she was also, most famously, Valeria Watt in Carry On Screaming. Valeria, you will remember, was the evil vamp who said "do you mind if I smoke?" as wisps of smoke rose up from her writhing body (after "infamy, infamy, they've all got it infamy", it became the most famous line in Carry On history). She was also the one who vitrified women and turned them into shop dummies - a mix of camp and creepy that's pretty hard to forget.

I should also tell you about some of Fielding's less famous work, such as the project she's just completed with the video artist Martin Firrell, which features the actress as a kind of purveyor of showbusiness wisdom. Or the could-have-beens of her career, such as the movie Federico Fellini wanted to make with her. Then there's the bizarre film in which she plays a villainess obsessed with Cumbernauld, or the "jewel in the navel of Scotland" as she calls it. The film was made in the 1970s to encourage people to come and live in the new town, which leads to an interesting thought: there may be people living in Cumbernauld right now because of Fenella Fielding.

And here she comes now, entering the foyer of the Soho Hotel in London in a long overcoat and tangerine jacket, hair, lips and eyes 25 per cent bigger than real life. The hair is Marie Antoinette and the eyes are framed by oversized false lashes that take several seconds to raise and lower. She's wearing a huge poppy brooch at her neck and over-sized rings on her fingers: big adornments on a tiny 86-year-old lady. One of Fielding's rules for life is that a certain amount of physical display can be alluring and she still sticks to that rule.

And then there's the voice: posh, elegant, flirty, and only slightly diminished by age. When Patrick McGoohan was making his dystopian television show The Prisoner in the 1960s, it was Fielding he asked to be the voice of the village - a kind of Guantanamo Bay for British secret agents - and it worked: Fielding's voice was perky and cheery and all the more sinister for it. Hers is the kind of voice that lingers over words and enjoys them, and at the Soho Hotel she gets her first opportunity when she sits down and orders a drink. "Lapsang souchong please," she says, pausing after every syllable for dramatic effect.

When the tea arrives - "and some little biscuits! Darling, how lovely!" - Fielding tells me about the appearance she will be making at the Boswell Book Festival in Ayrshire this Friday. The festival in Auchinleck, the birthplace of James Boswell, diarist and author, is a celebration of memoir and Fielding will be talking about the biographies that she loves. In particular, she would like to revive interest in Enid Bagnold, a writer best known for National Velvet, a novel about a racehorse that was later a film with Elizabeth Taylor. In her 1969 autobiography, Bagnold also wrote about her adolescence and her sex life in the early 20th century. "HG Wells was her first chap," says Fielding, "and she wrote so brilliantly and frankly without being disgusting and without being mawkish and it was, to my amazement, exceedingly interesting."

Fielding will share her love for Bagnold's work, but she will also be talking about her career as an actress and comic performer. She has never been an A-lister but in the 1950s and 60s she did some pretty ground-breaking work in comedy, writing some of her own material, appearing at Peter Cook's Establishment Club and starring in hit revues, including Pieces of Eight with Kenneth Williams, who played her brother in Carry On Screaming.

She also had a developing career in movies in the early 60s, but it was that Carry On film in 1966 that scuppered it. The movie may have been her biggest hit but it also trapped her in one frozen image, like the women she vitrified: the image of the sexy, naughty, campy vamp. Producers couldn't see past it and her film career never really recovered.

"It was bad for me in a way," she says of the film. "I did a film with Tony Curtis called Arrivederci, Baby! but I got that in spite of Carry On Screaming. I think I would have had better parts in straighter films if I hadn't done Carry On Screaming. It doesn't make me bitter about the film though - it's so long ago and it's kind of a famous film even now."

Before Carry On Screaming, Fielding had made one other film in the series - Carry On Regardless in 1961 - but she wasn't one of the regulars and Screaming was just four weeks' work in one of the busiest periods of her career. "It was taking the p*** out of Hammer Horror and the Addams Family," she says, "with me playing the Morticia character and it was dismissed by the press at the time - but not by me. I got a couple of wonderful reviews but it was otherwise seen as another piece of rubbish."

Fielding's strategy was to play the part straight and, although it's funny, it's also disturbing (she and her brother Dr Watt, played by Williams, kidnap girls off the street and turn them into mannequins). "When I decided to do it," she says, "I thought I'm just going to play it straight and so I did. If you look at it, I do play it straight and I'm very bossy with my brother. The only scene that could possibly be called sexy is the one with Harry H Corbett in which I say 'do you mind if I smoke?' and that was done in two takes. After we'd done the first take, I said to Gerry Thomas, the director, 'Gerry do you think we could do another take because Harry is not coming on strong enough to me?' and he agreed. And this time he did come on to me. But he had to be TOLD to."

Fielding is amused by that, but not surprised because she thinks much of her famed sexiness is a sort of unreal, camp sexiness that helped make her a gay icon ("I had gay men very keen on me very early on") but also prevented her being taken seriously by other directors and producers. She's relaxed about it all now, nearly 50 years later, and has happy memories of making Carry On Screaming - despite the fact she was working with Kenneth Williams, with whom she had a difficult relationship to say the least. In his diaries, Williams refers to Fielding as "madam" because of the tempestuous time they had on the comedy stage show Pieces of Eight. Fielding says they didn't get on because she was a challenge to his ego.

"Kenneth and I clashed because he didn't want anyone to be equal to him," says Fielding. "He wanted to be the only one and a few little creatures creeping round him. When he wasn't being difficult, he was lovely, but when he was, he was horrible. I got wonderful reviews for Pieces of Eight and Kenneth couldn't bear that - he actually got me in a corner and read out my good review and sneered at it. He quoted passages from it and sneered. And he also threatened me - it was frightening. The drift of it was 'you watch it'. Kenneth wanted to be top of the mountain with little people round his feet."

In the end, Fielding proved that she was able to fight back and could be just as focused, ambitious and histrionic as Williams. There's a sense of drama about her still - when she talks, she does so slowly and carefully and illustrates it with some balletic gesticulation - but there's also a steeliness and a focus on her self-interest; Noel Coward said of her that she was a nuisance and "required knocking on the head fairly regularly".

Fielding first demonstrated this steeliness when she was still a girl growing up in London and decided she wanted to be not an actress, but a journalist. "I wrote for the local paper in Edgeware," she says. "I was still at school but the newspaper came out and I thought I could do that. I had the nerve to walk into their office and they said 'well, what can you do?' They couldn't take me on the staff because I couldn't do shorthand, so I said well how about features? I said I could write about people I know in Edgeware and they said, ok, write something and if we like it, we'll print it. So that's what I did for about six issues."

Fielding then decided she wanted to act, but it went down badly with her parents Philip and Tilly. "My parents didn't like the idea of my being an actress - not in the least," she says. "As far as they were concerned, it meant ruin and floating face downwards down the Thames. I had to resist it but in the end I couldn't because they took me away from RADA even though I had a scholarship."

Instead, Fielding felt compelled to take a secretarial course. "I sort of sobbed into typewriters in people's offices for a while," she says, "and I wasn't the most efficient either. I endured it for a while though and then I started doing amateur acting work and got spotted by professionals so I decided that I was going to do it - I was going to act."

It did not happen right away though and Fielding spent several years struggling to make a living. She has never married but in the 1960s her boyfriend gave her a pound a week to live on. "My cousin Billy had luncheon vouchers and was on a diet," she says, "so she gave me her vouchers and my boyfriend at the time - it doesn't sound great - gave me a pound a week. So what with his pound and her luncheon vouchers, I managed fine. I suppose I was living on the edge of poverty but it didn't feel like that."

Eventually, gradually, she started to find success and found work on television in the 1950s and early 60s, appearing in episodes of The Avengers and Patrick McGoohan's show Danger Man, which later led to her gig on The Prisoner. She also attracted the attention of Fellini, when she was appearing in the stage farce Let's Get a Divorce. "He went wild about me and said he wanted to come to my place to see where I lived," says Fielding. "He came up and looked at everything and wanted to see me in my setting. I was a potential muse - he wanted to see all my photographs and he had a word with my hairdresser - it was extraordinary. He said he wanted me to do a film which would be about seven different aspects of a woman and I would play all the parts."

It sounds like a wonderful project but in the end it didn't work out. "I didn't actually reject Fellini. What I did was I took another job that was offered at the same time, but I do regret it because it might have been quite an experience. You never know, but he never asked me again because he felt that I'd rejected him by taking another job."

It is certainly interesting to speculate what might have happened had the Fellini film worked out - maybe Fielding would have had the serious film career she craved. From the start, she always wanted to do more straight work, but pretty much always ended up in slapstick comedy or campery, which means much of her career is made up of curios and oddities - such as the film about Cumbernauld.

Called Cumbernauld Hit, the film was sponsored by the town's development corporation and was made in 1977, while Fielding was appearing at the Edinburgh Festival, and it's a strange little thing. Fielding plays a Bond-style villainess and she and her accomplices are shot against the new, giant concrete flyovers of the new town. "Cumbernauld," she announces dramatically at the start of the film, "is tomorrow already here! A town festooned with awards for community architecture! A veritable jewel in the navel of Scotland!"

Quite how a weird film about a villainess organising a heist was going to attract people to live in Cumbernauld is unclear but Fielding loved making it. "I knew Cumbernauld was a new town and the film was to publicise it in a jeu d'esprit and at the time, it was quite experimental," she says. "I was there for about two weeks and I think Cumbernauld was possibly rather good." And has she ever been back there? "No," she says, a little sheepishly.

If you would like to see this strange episode in Fielding's career for yourself, parts of the film have recently been uploaded by Scottish Screen Archive and can be viewed online - which is largely where Fielding now pursues a quirky career-in-retirement. She has an internet radio show in which she plays her favourite music and the art project with Martin Firrell has also recently gone online.

Fielding tells me that the project with Firrell - who is famous mostly for projecting provocative statements onto the sides of public buildings - happened when he got in touch and said he wanted to create a piece of video art with her. The result is an interactive website in which the actress dispenses bon mots inspired by her career - and the best of them are little snippets on Carry On Screaming.

Valeria justified her killing, says Fielding in one of the clips, by claiming that she was preserving women in their youth, but that was a bad excuse. "Preserving your youth is a no-no," says Fielding. "It's a terrible waste of time."

Fenella Fielding is at the Boswell Book Festival on May 9 at 7.30pm. For more information, see boswellbookfestival.co.uk. Martin Firrell's project is at metafenella.com