Alison Steadman was born in Liverpool in 1946, where she went to a state grammar school and, as a teenager, sneaked off at lunchtimes to see the Beatles at the Cavern Club. She made her stage debut at 22 and went on to establish her name in television dramas such as her former husband Mike Leigh’s Nuts in May and Abigail’s Party, The Singing Detective, Pride and Prejudice, Fat Friends and Gavin & Stacey, which returns in December for a final Christmas special. She has appeared in numerous films including A Private Function, Shirley Valentine and Life Is Sweet and her stage roles have ranged from Madame Arcati in Blithe Spirit to Mari in the National Theatre’s The Rise and Fall of Little Voice, for which she won an Olivier award. Her memoir, Out of Character, has just been published.

In your new memoir, there’s a contrast between the Alison Steadman who is more than happy quietly watching the birds in her garden at home in north London, and these larger-than-life women that emerge out of you almost – Beverly in Abigail’s Party, Mrs Bennet in Pride and Prejudice, or Pamela in Gavin & Stacey. Is that contrast ever unnerving?
I love watching my birds every day, watching their world, what they’re up to. But then, you know, we’ve just been filming the last-ever Gavin & Stacey Christmas special, and it’s honestly been the best fun ever. And for that, you’ve really got to leap in and be this character, not hold anything back. I love that balance between the two.

You’ve been approached to do memoirs over the years and have resisted it. Why now?
I’ve never wanted to do it, because I don’t like to talk about my private life. I did this one now because I was assured it could be just about my career. Some journalists are so intrusive. I was even followed by a woman in Marks & Spencer’s food hall in Muswell Hill once, and she put it in the paper that I’d only got a cabbage in my trolley.

It doesn’t strike me from reading the book that you’re a very tortured soul?
I’ve had a very happy life – taken on the whole, obviously. My relationship with [director] Mike Leigh was good and we’re still friends. We’ve got two wonderful sons. I’ve been with my partner [actor] Michael Elwyn now for 28 years, and I’m really happy. And I’ve got two little grandsons. I’ve been away working on Gavin & Stacey for five weeks, but every time I get a photo of them I’m so excited.

You were the youngest of three sisters, growing up in Liverpool. You talk in the book about how your start as a performer came wearing your mum’s fox fur imitating [comedian] Hylda Baker. Would you also mimic your family members?
No, it was more people I’d seen on television or in school. I went to an all-girls grammar school, and we had some quite extraordinary teachers. I used to impersonate them in front of the class before they arrived, or I’d walk behind them and do their walk. I loved the fact that all the class would be laughing. I was on stage basically.

You’ve never lost that habit of observing people?
Sometimes I get cars to places if I’m working. But otherwise, I love being on the tube or the bus, because that’s life. I’ll sometimes take notes on my phone about what somebody’s wearing – red tights and black stiletto heels or whatever – so I can remember them when I get home.

You recall a breakthrough moment when you had been improvising, with Mike Leigh, the character who became Beverly in Abigail’s Party, but how she really came to life when you picked out the orange evening dress she wore.
Yes. I had a wonderful tutor at drama school, and one day she asked us all to pair up with someone and go off and swap clothes. I had this old woolly jumper on, and a pair of jeans. And the girl I’d swapped with, she was very busty and she had this fabulous tight top on. When I saw her in my clothes I thought: I’m going to throw that jumper away. It was a great lesson in why people pick clothes and how that helps make them who they are. I never forgot that.

Are characters sometimes gifts from real life?
If you play someone like Mrs Bennet, it’s there, it’s written, that she’s eccentric, she’s over the top, she’s panicking about her daughters. But I kind of based all that on my Auntie Mary, my mum’s sister. I couldn’t say that to anyone until my Auntie Mary had passed on, though, because she was lovely and I didn’t want to upset her.

Much of English comedy, from Jane Austen on, is based on class difference and aspiration. Are you wary of that fine line between parody and cruelty?
You want to be truthful, is the main thing. Like when I was playing Beverly, I never consciously thought I’m going to send this woman up. I was just being Beverly in my head, what Beverly wanted, you know, how she saw herself potentially as a model, but how she’s a lost soul, really, scrambling around looking for, whatever, the best leather sofa.

Did you ever feel trapped by class yourself?
I don’t think so. I’m still in touch with several people I grew up with in Liverpool. There was never a sense that I was coming from a rough and ready place, just a suburban house, three little bedrooms, a nice garden back and front. It gave me a good solid base for my life. After that, you can meet the Queen but still know who you are.

You write that you knew from the first lines of the first script of Gavin & Stacey, 17 years ago, that Pamela was made for you. Why was that?
You have to remember that at 20 I went from Liverpool to drama school – at East 15 – in Essex. I got surrounded by Essex people. I’d never had eel pie before. I think I must have absorbed all that. And from the start Ruth [Jones] and James [Corden] just nailed it in the writing.

It must be gratifying to know that at Christmas families like mine up and down the country gather to utter Pamela’s catchphrases – “Oh. My. Christ!” – around the dinner table…
I feel really proud. I mean, the last Christmas special got a huge audience. People stop me now in the street all the time: “Can’t wait. Can’t wait for Christmas Day!” I hope we don’t let them down!

In the book you mention how at school you were called Unsinkable Molly, after the heroine of the Titanic, always looking for positives, full steam ahead. Do you ever have a sense of slowing down?
Well, the week after next, I start seven new episodes of Here We Go, for the BBC, which I love. I’m not doing any more stage work, though, which is a shame, because it was my favourite thing. And life has changed. I’ve lost my parents. I’ve lost both my sisters now, and so you do find yourself thinking: well, you’re 78, how many more years? But then I think: Oh, just get on with it. You might get run over by a bus this afternoon. Be in the moment.

So: Unsinkable Molly?
I hope so.

• Out of Character: From Abigails’s Party to Gavin & Stacey and Everything in Between by Alison Steadman is published by HarperCollins (£25). To support the Guardian and Observer order your copy from guardianbookshop.com. Delivery charges may apply