When casting his latest film, the director Matthew Vaughn wanted his female baddie to be “absolutely an iconic beauty”. He knew exactly who it should be – a woman he had idolised since he was a teenager. So Vaughn got on a plane, flew to San Francisco, drove out to the chic, rural suburb of Palo Alto – all bookshops, alfresco Italian restaurants and expensive modernism – to the home of the woman whom scientists have described as the apotheosis of feminine facial beauty. She was knocking on the door of 50, taking a long break from work to concentrate on family life – a husband of 14 years and a teenage son and daughter – and keeping horses, miniature donkeys and lots of dogs in the countryside. The role he wanted her to take was that of the evil Lamia, a chillingly powerful witch desperate to find the fallen star (Claire Danes) whose heart holds the key to eternal youth and beauty.

“She’s a universal beauty,” says Vaughn of Pfeiffer. “Now, Angelina Jolie is beautiful, but some people think she isn’t all that. But I have never met anyone who doesn’t think Michelle Pfeiffer is gorgeous.” And I have to agree. Inside a lot of women, there’s a part that is jealous and small, a part that wants Sienna Miller to get fat, Jerry Hall to go bald, supermodels to be stupid and Jolie to take just one bad photograph and keep on with the charity-worker drab. Pfeiffer doesn’t seem to tap this vein, perhaps because she has an elegant indifference to her looks, which has seen her take as many ugly roles as beautiful ones.

We meet in a Palo Alto restaurant, and she is effortlessly pleasant. Slender and chic, she is dressed in a simple dark shirt, slim-fitting jeans (but not skinny – you couldn’t imagine her doing anything so vulgar and try-hard as fashion) and a pair of functional, not statement, sunglasses that she takes off as soon as she meets me. She’s hardly wearing any jewellery; her ears are peppered with homemade holes from a period of rebellious teenage piercing.

So what made her take this role as an ugly, old witch – a part that required her to look like a hag, not to mention pull the horrific visual gag of what time does to a woman’s breasts. For her, the indignity was overridden by “Matthew talking to me about a lot of nuances to the character that weren’t necessarily on the page”.

Because despite being a slapstick, OTT witch, the role of Lamia is actually a wrinkly metaphor for women’s battle against the ageing process. Vaughn says: “This character was inspired by all those women in LA who were once beautiful, and now look like freaks; the fact that the ageing process is scarier than claws and fangs.”

“For women it is!” says Pfeiffer, when I repeat Vaughn’s line to her. “The first time [I saw myself in prosthetic ‘old’ make-up], I literally gasped. I was so distressed, I ran into the bathroom to hide.” She says she looks like a monster, but to be honest, I have seen not dissimilar complexions on those once-beautiful, topless septuagenarians you spot on the beach in St Tropez. The key thing is that while this film deals brilliantly with the magical fantasy realm of Harry Potter, Narnia and its closest relative, the 1980s movie The Princess Bride, the card it deals on ageing is all too real.

“Matthew wanted to shine a light on that and poke fun at it,” Pfeiffer says. “To play with our obsession with youth and the ludicrous degrees to which women will go to reclaim it. Lamia’s desperate quest for youth [in the form of eating Danes’s heart] is a metaphor for the grotesque mutilation taking place in society.

“I don’t think anyone is going to be condemned for a little something done here or there, but people have lost sight of what’s beautiful. There’s a lot that you can do surgically and otherwise to make yourself look younger, yes – but not necessarily better. One of the most beautiful women I have seen in my life – still young and truly a beauty – I hate what I have seen happen to her,” she says of a well-known woman she will not name. “It’s like some weird anorexic disease where people don’t see what’s in the mirror.”

A sort of body dysmorphia, something that used to be a mental illness? “Right,” she says, “and now it is a disease of our culture. It just keeps growing. We have less and less to compare it to for our idea of normal. In fact, it’s really hard to even remember what normal is.”

Vaughn’s prosthetics people based Lamia’s ancient body on pictures of 90-year-olds doing yoga naked. “I looked at them and, well, we don’t look good when we get old,” says Vaughn. According to him, women who have seen the movie have “gone bananas” for the ageing horror-comedy played out by Pfeiffer’s character. “They say, ‘At least someone is addressing how we all feel.’ ” Ageing is the new bogeyman. “Look at all the stuff my wife promotes [Claudia Schiffer, who is the face of L’Oréal], I can’t believe it works.”

Pfeiffer says: “I found all this very interesting coming from a man who is married to a young, beautiful model. Someone who I would not imagine is feeling all those age issues yet, but who knows what plays out in the model world.”

Pfeiffer seems remarkably serene about the human body’s inevitable decline, but then she looks astonishing for a 49-year-old. Her eyes have natural creases around them, her nose is her own and there is none of waxy appearance of an overly lasered epidermis. “I don’t do that much to preserve. I used to worship the sun when I was younger – I’m a southern Californian girl, it was all baby oil and beach life – but now I get white spots, so I stay out of the sun. I really have to. And, you know, I read about some miracle product and think, ‘I should try that, it’s going to be great. I’m going to get that cream,’ and sometimes I go out and buy it, but I forget to use it after two weeks, or I get a rash.”

She shows me her nails, and they are all random lengths, a couple of them a bit grubby, no polish. “I can go months, years without a mani. I never pluck my eyebrows. The make-up artists shape them only when I am doing publicity. I don’t get my hair cut between films, just when I work and I have to. As far as body maintenance goes, I do eat well and I exercise. I go at it hardcore in my gym, but that’s it.”

As Vaughn says: “She’s ageing gracefully. People who age gracefully look so much better.” He says that he loved working with Pfeiffer, having admired her since her two breakthrough films of the early 1980s. “I loved Grease 2 when I was a pent-up teenager; I loved Scarface,” he says. “I was obsessed with her as a kid. She’s one of my top two all-time great beauties. No 1 being my wife, obviously – I have to say that.”

In the past, Pfeiffer has been quoted, like every other actress of her generation, complaining about the lack of decent parts for older actresses. Demi Moore, five years her junior, is rumoured to have spent £250,000 on youth-preserving surgical procedures and still says she struggles to get good roles in a youth-obsessed Hollywood. Post The Devil Wears Prada, Meryl Streep, 58, threatened to retire if the industry didn’t start producing better, more complex roles for women than the “dragons or gorgons” she describes as the norm.

“The whole idea of [Lamia] ageing as she loses her magic powers is an obvious allegory for not just the Hollywood system, but how women’s power is tied in with their appearance,” says Pfeiffer. In the days of Hillary Clinton and Condoleezza Rice, things may change – Pfeiffer acknowledges that as more women rise to power in the movie business, more interesting female roles are emerging.

“I’ve defied the obsession with looks in this industry and not allowed it to shape me,” she says. “I’ve always known beauty is fleeting; I have a fear of living in the past. I don’t have any awards I’ve won anywhere in the house, I don’t pine for some moment when I was at the top of my career, or a way I used to look. I try to live in the present. It’s a real trap in our industry – women who have the same hairstyle as when they were at their so-called peak. People get stuck in their time. I’ve spent most of my life not thinking about my looks and it has served me really well.”

Stardust is in cinemas nationwide from October 19

Times recommends

• Ingrid Betancourt's homecoming
• Geraint Anderson: Trust me, greed ain’t good
• The 10 best campsites in Greece