Pauline Collins: from Shirley Valentine to Dustin Hoffman

^"Surprises," says Pauline Collins. "That's what I love about this business. Even at my age, you can get surprises." The surprise in question was a request from Dustin Hoffman to star in Quartet, his debut as a director. "I thought, he doesn't even know me." And actually he didn't. But Maggie Smith and Tom Courtenay had recommended her, and Hoffman watched her give an interview on the red carpet for some movie event and decided he wanted her.

He phoned her and they chatted for two hours, and then he offered her the part. Collins plays Cissy, a former opera singer with vascular dementia living in a home for retired musicians, in this gentle, likable film. Cissy moves seamlessly between past and present, real and imagined, coherent and loopy. It's a terrific performance from Collins – poignant yet vibrant.

Before we meet, I'm sure I know what she'll be like – warm, fun, trusting. After all, she's played such characters for more than half a century, from parlour maid Sarah in Upstairs, Downstairs to dippy housewife Clara in No, Honestly, both in the 1970s; and, most famously, the liberated Liverpool housewife Shirley Valentine a decade later. Her characters have also tended to be more downstairs than upstairs.

We meet at the Guardian. She lives in Hampstead, just a few miles away, and offers to pop in. Collins has always seemed the obliging type. So we sit in the bar underneath the Guardian and she orders a pot of tea – now 72, she's teetotal, tiny and more refined than I'd imagined. When the tea comes, I start to pour, but she stop me. "You might leave it for a bit. It needs to draw."

Collins partly based Cissy on her mother, who also had dementia. "The difference between [vascular] dementia and Alzheimer's is that with dementia the person is left. You can see little windows; in Alzheimer's it's like the person has no soul. If you look in their eyes – it's like a tunnel." Collins is so different from those lovable ditzes she has often played. She's smart, steely and serious. She comes from a family of teachers (mum was a senior teacher, dad a headmaster for 25 years; she was a supply teacher in her early days, between acting jobs). Her sentences often finish with a clarifying statement or a question, to make sure you have understood.

"Now is this tea done enough?" she says. "I think it might be. Let's see." She sips approvingly and returns to her mother's dementia. "You enter into their world – rather than saying, 'No, it's not true, that's not your mother, that's your daughter.'" Does she worry about getting dementia? "I think all of us do because it's so prevalent." She says she helped her mother through her final years by embracing her past. "In her 80s, she wrote two volumes of her life story: her childhood and early marriage. I used to read it to her every day. And her brain and eyes geared in because that's the part they remember. So when it's her at four being told off by the teacher, she's, 'Yes, yes, yes.'"

Her family grew up near Liverpool, then moved to London. It's ironic that Collins has so often played working-class characters because at school she was taunted for being posh. She considered applying to Oxbridge, but got the acting bug and joined a troupe in Ireland. She was confident in some ways (intellectually), but not in others (she thought most girls were more glamorous and leggy than her).

Collins has had a fascinating career, not least for its absences. Every time she got a break, she didn't seem to use it. After appearing as Samantha Briggs in Doctor Who in 1967, she was offered another 39 episodes but turned them down. "I thought it was like a prison sentence. Maybe it would have given me a profile early in my career, but then I would have missed so many things."

In 1969, she starred in the first series of the popular TV comedy The Liver Birds, then moved on again. She gave Upstairs, Downstairs the elbow after two series. There followed one series of No, Honestly (in which she and her husband John Alderton played a married couple). She didn't get her film break until she was nearly 50, and when it came it was massive – she was nominated for an Oscar for Shirley Valentine. But not much followed: it's not that she wasn't offered parts, just that she didn't fancy doing Shirley Valentine-ish roles.

"I tell you what, we have always been movers on," she says of herself and Alderton. "Everybody has to do a series now and stay on for 10 years or whatever. But both of us liked to change after doing one or two."

I have a theory about Collins: that she didn't work for such huge stretches because she was determined to be a devoted mother to their three children. And I'm convinced this was due to a traumatic event earlier in her life. When Collins was 23 and working in Ireland, she discovered she was pregnant by a boyfriend she had recently split up with. She kept her pregnancy a secret from her parents, even though she knew they would be supportive; she took herself off to a convent where nuns delivered the baby, looked after Louise for six weeks, then gave her up for adoption. When Louise was 21, she wrote to Collins and they were reunited. It's an astonishing story, both heart-breaking and heart-warming.

In 1992, Collins published a book, Letter to Louise, a beautifully written account of her childhood and early adulthood leading up to the adoption. Some of the most tender passages come when she is in the convent with other girls who had got themselves into trouble. "I remember the last time I saw you," she writes to Louise. "We were about six feet apart ... Every day of my life, I've relived that moment, replayed each second like a book of flicker pictures, clinging frame by frame to the last images of you … Why did I give you away? Now in 1992, I still feel a blow in the solar plexus when I consider that question. I feel as if my soul is punched out through my throat. Now in 1992, I cannot understand why I did that terrible thing, why I didn't look harder for another solution."

But when I mention Louise, Collins freezes. "Now she likes to be mentioned but not discussed," she says firmly. Is she close to her? "As I've said, she likes to be mentioned but not discussed."

"Can I ask a question without talking about her?" I say. "Did you continue working during this period?"

"That involves her, so, no, I won't answer that."

I try to think of a way round it, but I can't. Everything comes back to her children. What do her other kids do? "All of my children like to be mentioned and not discussed," she repeats.

So we chat about Hoffman and how great he was to work with; how occasionally she had to remind him she wasn't playing his autistic character in Rain Man; how she was bombed out of three homes when she was little; how she recently played a real meany (in last year's Albert Nobbs, alongside Glenn Close). But nothing seems quite right. The tension's palpable.

I don't get it, I say. If you refuse to talk about Louise, why did you write a book about the adoption? "That's the only question I'll answer," she says. "Two journalists were researching it and I thought, I know that story – and they don't. So that's the reason." A bit of you must have wanted to write it? "I would have written it privately, let me put it that way. It would not have been for publication." So it was already in your head? "I'm not going to answer any more questions on that because you're a sneaky bugger."

She and Alderton have been together for 43 years, and married for 42. There was a time when it was suggested they could have been the new Burton and Taylor. She laughs. "I don't think so. Very flattering. If only." She has a point: the pair would always have been a suburban Burton and Taylor.

In one way or another, family has always been at the heart of her life. There's something sad about the fact that she feels unable to talk about Louise and the adoption, especially when the story has a happy ending. But one thing is clear: that period of her life is largely responsible for who she is today. And that person insists she doesn't have time for misery or darkness. Not even when she's playing somebody with dementia.

Occasionally, she's been offered heavy parts and has tended to turn them down – notably, a role in a film about the British serial killer Dennis Nilsen. "Dreadful!" she says at the thought of it. "It's bad for the spirit to do stuff like that. If it's going to be worthy and hard labour, I'm not going to enjoy it and the audience won't either. I want to enjoy what I'm doing, especially when you're this old, and who knows how much life is left?"

Quartet is out on 1 January.