Abigail is having a 50th birthday party? The actress who personified the worst of suburbia is not looking forward to old age

Alison Steadman is not at all happy about turning 50. "I hate it, but there's nothing I can do about it," she says gloomily. "I don't feel 50, I don't know what being 50 should feel like. I liked being 40, life begins at 40 and all that. But 50 - menopause, old age, creaking bones, it seems to me you're on a bit of a downward slope. You're not climbing any more, you're on the cusp and you're going to go down."

Oh dear. Somehow you'd expect something a little more buoyant from the creator of some of the most memorable and effervescent female characters of British theatre and television. Pride and Prejudice's Mrs Bennet would spit in the eye of 50. The ghastly Beverly of Abigail's Party would greet 50 in an unsuitably tight cocktail frock and diamante at a huge birthday bash with canapes and sparkling wine. Alison Steadman's current creation, pillar of society Mrs Maria Helliwell in J B Priestley's comedy When We Are Married, is most definitely no spring chicken, and she is joyfully celebrating her silver wedding anniversary (though admittedly the festivities go sadly awry, with hilarious consequences).

But the actress herself looks genuinely sad; though 50-year-olds up and down the land would probably kill for a few of her natural advantages. Her roles are rarely even remotely glamorous; she has run a gamut of harridans, harpies, trollops and old bags. But her face in repose is lovely, sparingly made- up (no Hollywood-style mutton-dressed-as-lamb here) with perfect English- rose skin and deep grey eyes. You could hook a coat-hanger over her cheekbones; she has beautiful, slender, expressive hands. And she also has a dreadful, croaking cold, and she's just spent half-an-hour stuck in a Tube hold- up, though these factors are probably not quite enough in themselves to account for such gloom. She has not had a happy year so far; apart from the horrors of half a century, she separated from Mike Leigh, her husband of 23 years and the director whom she worked with on some of her most famous roles. This is something she politely but firmly refuses to talk about ("it's difficult and it's painful, not so much for me and Mike but for other people involved").

But when she gets on to the subject of her current role, she sparkles. She is sitting in her little dressing-room backstage at the Savoy Theatre, which is just as a dressing-room should be, with a huge mirror, bright lights, a great jumble of make-up, and several huge bouquets jammed on to the dressing-table. She prefers theatre to film, she says. "It's more fun, more exciting. It's hard work to keep repeating the same play and keep it fresh. But it's nice to hear the audience respond, and every performance is different, every audience is different. With the theatre, when you've finished a show, you're ready for a night out; you feel exhilarated, the adrenalin carries you through. After filming you're just like a rag doll."

Priestley is evidently therapeutic; the play, set at the turn of the century, is a very jolly comedy, and it's obvious that the cast are getting at least as much fun out of it as the audience. Maria Helliwell has a definite touch of Mrs Bennet about her; there is plenty of opportunity for sweeping about, squawking and vapouring - though, says Alison Steadman, strictly in a timeless kind of a way. "It's the essence of the character that's important. Jane Austen draws all her characters so brilliantly, so clearly, and Priestley does as well. They're very believable. The thing I always think is that if you can move a character from the period it's set in and imagine that woman living now, that proves that they are very real. Mrs Bennet I could play totally 'now', and Maria Helliwell exactly the same. You can see those kind of women living and breathing today."

There is a slightly worrying feel to the notion that such women-living- and-breathing-today might be innocent passers-by; that a lollipop lady, say, or an assistant in Boots, or, erm, a journalist, might suddenly find her most unattractive foibles immortalised in a new Steadman creation. Meeting her, there is initially a strong instinct to sit very straight, refrain from fidgeting (or, indeed, any unnecessary movement at all), and restrict all conversation to strict necessities, delivered in a clipped and businesslike tone (though not, of course, too exaggeratedly clipped and businesslike). In fact, though, there is absolutely nothing to fear. (Well, that's what she says, anyway.)

"People get worried when they're with me that I'm observing them, but it's nonsense really. I don't sit there and see a certain characteristic on someone and think 'oh, I'll use that'. It's not as simple as that. You've got to base your character on a real person that you know. It's a bit like making a sketch; you don't start off by drawing their nose hairs or a pimple on their lip. You start off with a broad outline, then gradually fill in the details. It's exactly the same when you're building a character. You can't start with their tic, or their twitch, or the fact that they don't like bananas, you start with the general feeling of them as a person, you build up a picture very slowly and add to it."

But those real people that she knows, the ones who ended up as ghastly Bev, or weedy vegetarian ditherer Candice Marie of Nuts in May, or Wendy, the ultimate suburban mum of Life Is Sweet - does she still know them? Because, while in many ways being immortalised forever would be extremely flattering, you would have to think long and hard, in a terribly positive frame of mind, before you would feel flattered at being immortalised by Alison Steadman. After all, her forte is distilling the world's most frightful, maddest, craziest, most ridiculous and pretentious friends, relatives, acquaintances and enemies into a single 100 per cent proof performance.

In fact, she says, she hasn't lost any friends. "People think they see themselves all the time, they think they know who it is. People come up to me and say, 'oh, Candice Marie was based on me, wasn't it?' or 'it was based on so-and-so, I know that', and they're always wrong."

Her scalpel wit makes audiences cringe as well as laugh (there can be few girls of formative age who have watched Abigail's Party without swearing a secret, internal oath that they will never, never, never let themselves grow up like Beverly). This wince factor has led to her being described as "cruel" and even "brutal", charges she denies emphatically. "I set out to entertain, to amuse, and part of that becomes something that's being said about our lives, and our society. But I'm not setting out to be cruel and brutal, I'm setting out to reflect life, put a mirror to it, by creating characters and saying 'look!' Beverly, for instance: taking a very naive look at it, you'd say she was a very superficial kind of person, and it's cruel because those kind of women are innocent victims, they don't really know they're being like that and why laugh at them ... well, I think that's all nonsense," she says robustly. "What is comedy? It's life. You laugh at someone else's misfortune, like the guy who slips on a banana skin, or someone else's foibles. If we were all terribly wholesome, portraying people who are upright and safe, it would be so boring. You can make people laugh and move them and make them think, all at the same time."

Her faith in her ability is borne out by the longevity of her creations - amazingly, Abigail's Party was first televised 18 years ago. "My eldest son was 18 in February, and I was five months' pregnant with him when we recorded it for television. So that's how I always know. People often say to me 'I was at school and my mum let me stay up to watch it,' and I think 'oh my God'. I don't get fed up with Bev, though I just occasionally wish people would remember something else as well. But it always comes down to Candice Marie and Bev."

It seems that this frightful twosome are both still alive and well and living - absolutely everywhere. "I constantly meet people who say 'I know someone exactly like that'. It's like a revelation that there are people walking round who are like those people. After all, creating a real character is what I set out to do. And men: it's a constant succession of confessions about 'Beverlys I have been kneecapped by', all these poor guys and these awful man-eating women who suddenly say 'right, I'll have that one'."

She is fascinating to watch; she makes faces, mimics, slips in and out of different accents and grimaces. Her natural speaking voice still retains traces of her Liverpool background; she came to drama school in London at 19, after a stint at secretarial college which she "loathed" and 18 months working in a probation office ("it was wonderful, all life was there"). She joined a youth theatre, worked in rep, and was already well on the road to success when Bev in her hostess gown sashayed into her life.

Somehow she fitted in two sons, now 15 and 18, who have sailed happily through drama GCSEs and Theatre Studies A-levels (they like her work with Rory Bremner and her voice-overs for the satirical animated series Crapston Villas, of which there will shortly be a new series, but "anything else is boring, they don't even bother to watch".)

She hasn't changed much over the years, she says; she lives in London, travels by Tube, buys her jumpers at Marks & Spencer. That (damn) brings her back on to age. "The main thing in life is not to close any doors, which is why 50's a bit awful because you think that it's a slowing down process. But I'm not going to let that happen. You've got to keep on discovering," she says. Oh, for goodness sakes, enough already! "Over the hill" and "Alison Steadman" are two phrases which have nothing whatsoever to say to each other.

8 'When We Are Married' is at the Savoy Theatre, The Strand, London WC2 until 23 November. For details and booking, telephone: 0171 836 8888.