"Do you know Paris?" The question is shamefully inconsequential, a piece of conversational putty while I am trying to process Dame Maggie Smith's wicked, but entirely truthful, admission that she cannot remember much about her latest film, except that it is set in Paris, and that as usual she plays the part of an old battleaxe.
Paris! It is as though the lid has been lifted on a box of snakes. Her memories of being ambushed produce a frisson of displeasure. She would love to have drifted through the city, visiting museums and art galleries, but it was impossible.
• Read more: Movie review: My Old Lady
"I was besieged by Americans and Downton Abbey," she recoils. "That's never happened to me before. It's television that does it. It was awful. I love wandering around on my own and I just couldn't."
Even in England, the consequences of becoming a television celebrity (more shivers) through her formidable impersonation of Violet, Downton's Dowager Countess of Grantham, can be unnerving. "I don't go to places and if I do I nearly always have to have a friend. It's very difficult when you're on your own because you have no escape."
Unless she's quite rude?
"That doesn't get you very far, although I think I am sometimes, because it can be quite scary, you know, if a lot of them come at you." The jostling curiosity of a herd of bullocks comes to mind. How does she deal with it?
"Run away mostly. I just make a beeline and go, go, go. It is hard. I don't know how people cope with it. What do they do, these huge movie stars? What the hell do they do? Perhaps they never go out. I certainly don't think they walk around on their own. If they want a word of advice, don't do it in Paris."
Her voice carries a withering sort of incredulity with echoes of Violet Crawley ("What is a weekend?") and Harry Potter's Professor Minerva McGonagall. This is vintage Smith, refusing to put herself in the stellar cast where she so obviously belongs, deliciously affronted by vulgar curiosity and yet fully acknowledging both the privilege and the absurdity of being in demand at the age of ... we might as well say 92.
"I'm always older than God in these parts now," she says with benign resignation. In 1991, when she was playing Granny Wendy Darling in Hook, Steven Spielberg asked a friend of hers in the costume department: "How old is Maggie Smith?" "My friend said without hesitation: '92'. I've been that ever since. They don't need to make me up any more, I'm afraid. I've caught up with myself.
"I was doing a scene with Penelope Wilton [the righteous Isobel Crawley in Downton] the other day and I got up with my stick and started wobbling around. 'Why am I acting old?' I said. 'Why am I doing this? I am old!' I'm trying to work it out: if I'm my age, the Dowager Countess of Grantham must be about 110 by now. She has become a monster.
"Penelope and I have a great time. We play endless Bananagrams. She makes me laugh a lot and we talk about the books we read."
In My Old Lady, the film written and directed by Israel Horovitz, Smith (actually 80 in December) plays a genteel but rather decrepit English widow of 90, rattling around in a vast apartment in Paris with her daughter (Kristin Scott-Thomas).
A feckless New Yorker, Mathias (Kevin Kline), comes to turf them out and claim his inheritance, unaware that under ancient French law the old lady is a fixture. Mathilde Girard is a cunning old bat, astute and mischievous but in the end made vulnerable by the secrets of her past; a perfect vehicle for Smith's ability to move and amuse at the same time.
Out of a pile of scripts awaiting her attention, she told Horovitz, his was the only one that didn't end with her character dying.
The production team were amazed at her transformation to the ancient Mathilde without benefit of wig, makeup or props - as well they might be because in person she is a very unconvincing 79, not in the least crumbly or imperious but slight and chic with one of those soft complexions that reminds you of the last rose after the first frost: it doesn't fully open but neither does it fall.
Smith is untouchable now - typecast, yes, by virtue of her age, but sharp as a tack, richly unpredictable and beyond censure. She can turn an ordinary sentence from dross to gold ("Lunch is not the sort of meal that interests me"). By what alchemy is unclear. Julian Fellowes, creator of Downton, gets close: "Maggie has this quality of imbuing every line with a wit and a dimension it sometimes does not deserve. For a writer this is an attractive gift in an actor. She also has a unique sense of comedy, based on a somewhat ironic view of real life, making it both funnier and more sad."
She looks as insubstantial as a leaf against the swanky sofa in the Goring Hotel's Royal Suite. Her long bony fingers test the strange nap of the upholstery.
"Teddy-bear, isn't it?" she smiles.
The reason she seems hazy about My Old Lady is that roles have been coming thick and fast. "It seems so long ago and so much has happened. I'm trying to remember whether I did it before the thing in India [Best Exotic Marigold Hotel 2]. My head's confused with that and Downton Abbey. It's all been merged. It's feast or famine in this profession." Although life without work would be alarmingly shapeless, she feels she has been working too much recently.
"The curious thing is I've been doing things that go on and on, like Harry Potter and Downton. I need to stop and take a breath - but I can't."
She is about to reprise her eponymous role in The Lady in the Van, a film version of the play Alan Bennett wrote for her and which she played in 2000.
She calls Harry Potter her pension. It helps to educate her five grandchildren. "I'm on my own, so it's lovely to be able to help my sons."
Smith has been famous for 60 years, but with nothing like the worldwide reach she has now in the autumn of her career. She made her debut with the Oxford University Drama Society in 1952. In 1963 she played Desdemona opposite Laurence Olivier, but as a young actress she felt inadequate and visually wrong. "I think there is an accepted way that a face should be, and I'm not like that."
Not that the lack of conventional beauty held her back. She won her first Oscar for The Prime of Miss Dame Maggie Smith shows no signs of slowing down as the mischievous Mathilde Girard in My Old Lady, and the indomitable Dowager Countess of Grantham in Downton Abbey.
Jean Brodie in 1969, the second for California Suite in 1978. Honours and awards (including five Baftas) have been flowing ever since.
She claims she has never had a career plan or an ambition to play a specific part in her life. "It's what turns up, quite honestly. When I started out I didn't have any of this [film and television roles] in mind. Not a scrap of it. I just thought it was going to be all theatre and wonderful."
She finds film-acting a test of endurance. "In the theatre you knew where you were at. You knew you had to go there in the evening or for the matinee and you had a length of time to prepare yourself and then you did it. But this is like being on demand every minute of what seems to me to be 12-hour days or more. Schedules seem to get shorter and shorter.
"Wherever I got the idea that working in film or television would not be so stressful as the theatre I do not know. The theatre might not be exactly a holiday, but it's certainly not as exhausting as film can be. It's an incredibly tough life.
"Bette Davis was right: old age is not for sissies. And old age is not for television because it's so relentless. The young are terrific, but even they get tired.
"If you have a terrible night in the theatre, you think 'tomorrow it'll all be wonderful', but you can never put right a thing that's forever on film."
Probably just as well, she decides. "Because there would always be something I wanted to change. There would never be a take that would be right. You'd go raving mad." The dangerous intimacy of television bothers her. "People are in a room with you, aren't they? I suppose they feel they know you very well. In the theatre you were remote. There was always a kind of distance, more mystery. You could be a very different person. This is really [she slaps the palms of her hands together close to her cheek] in your face. It's not a whole lot of fun."
The demise of the repertory system, she fears, means that actors miss out on essential building blocks of the profession. "I find it sad that people have to go straight into television and film now. Lots of them are thrown into a West End production without having the grounding. It must be sooo scary."
Last year she received an Evening Standard "icon" award for her theatre work, but sounded regretful that she had done so little to earn it. (After getting over breast cancer in 2008, she seemed to have lost the nerve for theatre.)
"Everything's an icon," she says. "If you have been around long enough you are an 'icon'. A rather dusty icon ... or 'a national treasure'."
I have never met an actor with less belief in legacy - her own or anyone else's. After the obituaries and the tributes, fame vanishes. She thinks of her recently departed friend Lauren Bacall, and of Robin Williams, extravagantly eulogised, quickly forgotten.
"It doesn't last. The world is replenished all the time. I don't think people will remember."
She keeps no diaries, writes no memoirs and was "in no way connected" with the biography written about her in 1992.
"It's ceaseless, all these books people write. There's no mystery left to anything."
Her sons, Chris Larkin and Toby Stephens (by her first marriage to the actor Robert Stephens), are actors. "God knows, they weren't encouraged. I think they sort of fought it, but I'm glad they gave in because even with all its horrors this profession is fascinating."
Since the death of her second husband, the playwright and screenwriter Beverley Cross, in 1998, Smith has lived alone. Work stops her thinking about loss but when filming comes to an end, as now, it creeps back.
"They say it goes away but it doesn't. It just gets different. It's awful, but what do you do? After the busyness you are more alone, much more. A day that is absolutely crowded keeps your mind away from why you are alone but when it stops there is that deafening silence."
Although she is supposed to be having a month off, she is plotting how to fill the void.
"I shall be dedicating myself to Alan Bennett and The Lady in the Van. Learn, learn, learn. Mercifully it does look familiar."
She has no difficulty in learning lines - but touches polished wood in the hope of never needing "one of those awful things they shove in people's ears" - a permanent prompt.
"Can you imagine?" she says. "You'd have that rather distracted look."
And she's away, mimicking the vacant American actress who, it is said, opened with "Good morning, everybody. Move down left of centre," instead of her lines.
It doesn't look funny written down, it isn't particularly funny, but we both collapse with laughter. "I'm sure it's apocryphal but I just love it."
Smith is a genre all to herself.
What: The film My Old Lady at cinemas now
Also: Downton Abbey season four, screening on Prime 8.30pm