I feel loved in London

She said it was the first time she had seen the play from the mother's point of view rather than Tom's, and his desire to break away and be free. Maybe that's it - it presents itself at a certain time in your life."

Lange's reflections on motherhood have been prompted by the fact that all three of her children - Shura (by Mikhail Baryshnikov), Hannah and Walker (by Sam Shepard) - have left home.

"It hit me hard. Some people I know were joyous: now they could have freedom and do what they want, not be a slave to the school schedule. But," she laughs, "that kind of gave meaning to my life. Now I'm not so sure what I'm supposed to be doing."

While Lange is in London she's staying in Kensington, next door to a flat that is being renovated. "How can people afford to buy houses here?" she asks, exasperated. "I look at the prices, and I have to remind myself that this is in pounds. Which means it's basically double. So if you're looking at a flat that's £5.6 million, you're talking about $10 million. Hmmph."

It's part of the London condition, she has decided. "You have to be rich to live in London. Don't they say that no more than a quarter of your income should go on accommodation? It must be a lot more than that here."

Since she arrived her focus has been on the play. There has been no socialising, only work - when I speak to her she is about to be whisked off to appear on Richard and Judy.

But she has a large network of friends in London, from two previous stints on the stage here, playing Blanche DuBois in Williams's A Streetcar Named Desire in 1996 and as Mary Tyrone in Eugene O'Neill's Long Day's Journey Into Night in 2000. Among these are the Stoppards - it is Tom's son Ed who plays her character, Amanda's son in the play.

Her own family, she says, are fans of the play, and are making detours to come and see her it.

"Sam saw it in preview, my daughter saw it on the opening night, and my other two children will be coming during the run. They liked it very, very much."

She certainly doesn't look like a grandmother of 57, even in her rehearsal clothes: a dark cardigan and jeans, and black lace-up shoes of the kind that might have been favoured by Miss Jean Brodie. Even in mufti, she exudes a sense of coiled magnetism. There is a blur of ink on her hand, a tattoo from her spent youth. I ask how she handled the transition from bohemia to motherhood.

"By the time I had my first child I was already 31. I had really flown through the Sixties in my twenties, and had lived pretty much as hard and rough and crazy as I could sustain. It was time for me to slow down.

"Coming out of the Sixties, it was quite insane. It was completely peripatetic, never living anywhere - literally living on the road year after year. Moving to Spain, then to Paris, and then back to New York, and the whole underground arts scene. Not to mention the drug culture." She laughs. "There was a lot going on. The ones of us that are still alive, we're lucky we made it."

Drugs, she says, were "just part of life", and she is reluctant to specify how far her experiments went. "Not as far as a lot of people that I saw come to an end. By the time I hit my thirties I was really ready to have some responsibility. And to have that thing that connected me to life."

Lange's rootlessness wasn't necessarily rooted in the counter-culture. Her childhood was a tour of small towns in Minnesota. "My dad was very restless, so we'd stay in some little nowhere place. [Bob] Dylan described Hibbing, which is where he's from, as a town that was going nowhere. I lived in a lot of those places.

"I remember having a yearning that was so powerful it was almost like a physical pain. This yearning to get out, to see something, to do something."

She enrolled at the University of Minnesota as an art student, fell in with a group of photographers, and set out for Europe.

In Spain, her group documented flamenco gipsies. In Amsterdam, they filmed the life of a street person. In New York, she burrowed deep in the underground. Then she moved to Paris to study mime under Etienne Decroux.

"The first time I saw Paris - sounds like a song, doesn't it? - was May 1968, when we were coming from Spain on our way to Amsterdam, and the streets were like, wow! It was most thrilling thing in the world to me.

"The city was under siege. It was as close to a revolution as anything I've ever seen. So I thought, 'This is where I want to be!'"

In the midst of her European travels, Lange married her photography professor Paco Grande. She didn't take marriage seriously.

"It's never meant that much to me, the idea of marriage. I'm not married now, but Sam and I have been together for 24 years. So what does that mean?"

I wonder if monogamy has anything to do with it. "Certainly when I was with Misha, and now with Sam, they're monogamous relationships. Cos we have children. It seems like the sane thing to do."

Lange met Shepard on the set of Frances, the 1982 biopic of Frances Farmer which earned her an Oscar nomination.

She once said, "No one compares to Sam in terms of maleness", but bristles when reminded of the quote. "I hate talking superlatives. There's obviously something. I've been with the man for 24 years. And I'm still crazy about him."

Shepard's plays use the West in the way Tennessee Williams used the South, but he is less political than Lange, who considers America to be at "at a low ebb" because of Bush's foreign policies. Yet she is no fan of Hillary Clinton.

"I know she's a good stateswoman and she's incredibly smart. But I don't think I could support a candidate who supported Bush's drive to war."

Beneath her slightly reticent and respectable façade, Lange is clearly still a rebel, driven by a revolutionary spirit. To her, the heart will always rule the head, in politics as much as in love.

"With Hillary I get the feeling it's all politics. I would love to see somebody passionate and not scared, not always deliberating: 'Is this the right move?' But with some real sense of ethics and not afraid to go against the f***ing focus groups. That's what's killing films, that's what's killing politics."

Despite her Oscars - for Tootsie and Blue Sky - Lange's Hollywood career has been understated.

"Oh, I think Hollywood just got rid of me!" she says with a laugh. "Look, I had a chance to do a lot of really wonderful parts. But in 30 years, how many movies have I done? 25? For the most part, I like the work I did. I liked the experiences that I had. But if I could move on to something else, I probably would. I'm just not sure what else I can do."

Recently, Lange has revived her interest in photography. She beams at the mention of Rene Burri's photographs of Che Guevara, whom she recently described as her hero.

"This is the power of photography, isn't it? The iconography of Che Guevara really has to do with those photographs. I'm in awe of the revolutionary spirit. What an amazing journey that kid went on. Can you imagine coming into Havana with Fidel? I mean, God! What a thrill! There's nothing more thrilling - a revolution that works!"

Right now, her concerns are more prosaic. She is looking forward to a day off. "Hopefully I'll get out," she says wistfully, "and walk around. It'll be nice to get through this week, then you can start having a life. A little bit of one, at least."

Before she goes, she pays tribute to the play's director, Rupert Goold, and to the rest of the cast. "It's been a pleasure," she says. "It's been very good for me, to get me out of my head."

What, I ask, was wrong with her head? "You just don't wanna spend too much time dwelling on things!" she says, laughing. "It's like my father always said: 'You've got too much goddamn time to think!'"

The Glass Menagerie is at the Apollo until 22 May.