Rampling has been there. We know that. She has lived to tell the tale, or a version of it at least, in Andrew Haigh's chamber drama, 45 Years.

"That is the state Kate is in, after she's tried to rationalise things. She's been trying to keep it together, but suddenly she just can't."

Kate is a former teacher who has been married to Geoff, played by an immaculate Tom Courtenay, for the eponymous 45 years. The film opens with their anniversary celebration. She is both poised and practical.

"I've certainly known Kates, those types of women," Rampling says. "They are wonderful spirits, those middle-class English women who help out. They do charities, they have nice homes, they are there for people, but they also have a lot of things to do in life. My mum wasn't like that, but people around her were.

"And I say English, because French women are very different. And I was born and bred English," she lengthens the word ironically – Eeeeeenglish – "and I recognise it."

All seems well in this robustly ageing couple's tea-filled, dog-walking, child-free life, until a letter arrives for Geoff from Switzerland. After 50 years, a glacial melt has revealed the body of his former fiancee, Katya, who died in a climbing accident. Geoff has never told Kate anything about Katya. Even now, he doesn't tell her that Katya died pregnant, even as he drifts back into the past to the point before the life he never had. This taints everything, Kate tells him. Everything is ruined. But how can 45 years of marriage be erased in a single stroke?

"Of course it hasn't tainted everything. Of course they've lived all these years," Rampling says. "It's an illusion, but illusions can be as powerful as reality."

Jealousy is an animal response to an outside threat, she says. It doesn't need to be rational. Nor is there a healthy way to deal with that threat.

"In all situations, you either confront what faces you or you kind of negotiate it," she said in an interview with Indiewire, "but can you negotiate a couple's situation? There is no good way, so you choose a way. You must blast your way in and sometimes the situation may blast you back out … but you have to move it in some way."

And that, she believes, is healthy. Kate, having made the kinds of accommodations a long marriage demands, harbours anxieties she didn't even know were there. "She needs to go on a journey. She's got to unravel, and that can only be good, because that means you grow up. It's actually about keeping alive.

"It wakes you up. Maybe not to things you think are very nice because you are suffering, but through the suffering, you find … Oh dear, here we are, we're getting a bit religious, aren't we?"

Suddenly, she is shouting theatrically. "Through the suffering, you will be reborn!" She splutters into a piercing laugh, and her voice drops back to its usual low silkiness. "Yes! That lesson's over."

It's a curiosity, that laugh. It doesn't seem to suggest amusement as much as surprise – what have I just said? – which may explain Rampling's recent ill-judged response to questions about racism in the Oscars. Her assessment on French radio of protests against all-white nominee lists as "racist to whites" got her into trouble. She apologised and clarified, but it didn't help.

Charlotte Rampling is now nearly 70. Online discussions called her "elderly and ignorant", part of the "elderly Establishment", an irrelevance. It was an instructive example of how easily a national living treasure, if that treasure is a woman, can be rebranded as an old bat.

But then there is the success of 45 Years, which won her a Silver Bear as best actress at the Berlin Film Festival last year and now her first Oscar nomination. All this is clearly unexpected.

"It started with the critics, who were the first ones to push and push and push it," she said after her Oscar nomination. "They were so incredibly involved with this film, they got it. Then the audiences followed along."

Usually, she says, you assume just a few people will see a film such as this one: small, intense, raw but ultimately inconclusive. "But it's much more than that. It really is finding its place in the world."

As it was finding that place, however, Rampling was dealing with her own grief: her partner of 17 years, French businessman Jean-Noel Tassez, died in October. Rampling's response was to keep working, keep doing interviews, keep promoting this film about coupledom.

She has always dealt with life that way, working steadily after her sister took her own life, throughout years of depression through the 1980s and early '90s and then, after her depression lifted, in the years after her husband, Jean-Michel Jarre, left her for a younger woman.

"I suppose I feel that if one day I say, 'All right, I'm not going to do any more', I'll just drop dead," she says. "Not of boredom – I've got lots of other things to do other than filming – but it's what keeps me in the world. Even talking to you. I'm awake, I'm alive, I'm talking, I'm exchanging ideas with you, I'm thinking things, I'm meeting people."

We spoke at the Berlin Film Festival last February. If Tassez was ill then, she gave no hint of it. She laughed about his response when she said, for the umpteenth time, that she thought she might stop acting. "He said, 'You have a God-given talent and, if you don't use it, you will be severely punished'."

More religion, she agrees with another of her squawking laughs.

"'What are you going to do, anyway?' he asked. I said, 'I'll look after you'."

He groaned, which makes her laugh now. "Because I'm not a Kate. I don't look after people, not in that way. I do in my thoughts."

Anyway, she says, she doesn't want to be punished. "So now I'm having to wake up every day and think, 'Well, what am I going to do next?' "

A good deal, it seems. At 17, having been schooled expensively in England and France, she glided from modelling into a small part in Richard Lester's The Knack ... and How to Get It. Since then, she has made more than 100 films, most notably the scandalous The Night Porter with Liliana Cavani in 1974, and the series of films she has made with French director Francois Ozon, beginning in 2000 with Under the Sand.

But she never made films back to back, she says. "It seems when you look at a career that has gone on for a lot of years that it's a lot, but I didn't do film after film. I didn't have the energy, I don't think. Nervously, I couldn't cope. It's tough stuff, filming, not just doing the filming, but the whole thing. It takes you on a long journey."

So she travels in between films, goes walking, recuperates. All the characters stay with her, however. She can talk about anyone she has played, even half a century ago, as if they still lived next door.

"I like them," she says. "My ghosts, my friendly ghosts."

She gives another of those laughs.

"No, they're all right. They hang around, happy."

45 Years opens in Australian cinemas on February 18