A rare audience with Mario Testino

He's the world's most sought-after photographer, the man the A-list calls when it really wants to look its best. But what exactly is it that makes Mario Testino's portraits so alluring?

It's a Friday night in February and, in Tite Street, Chelsea, a party is in full swing: writers, journalists, academics, poets, TV people, fashionistas, a handful of society beauties. The hostess has encouraged her guests to bring their children, and the party is thronged with effortfully cool young teens. Especially eye-catching is a bevy of six-foot-tall teenage daughters, 15 or 16 at most, who totter about like Russian supermodels. Then a new guest appears.

His hair is receding a little, but is a convincing shade of chestnut. His features are blandly handsome. He has a dazzling smile. And as he smiles, a frisson of delight goes through the ladies in the room, for the world's greatest alchemist of female beauty – the man who makes everyone look beautiful – is in their midst. As though drawn by magnets, one by one, the literary mothers seize their coltish daughters by the hand and make a beeline for him. He extracts a pair of black-framed spectacles from his breast pocket and inspects each blushing ingénue before him, nods and says, "Aaaah..." to acknowledge her loveliness.

Each mother sighs in return. For, without a word being said, a thrilling instant of understanding has passed between her and the chap in the specs: that her daughter might, any day now, deserve to be photographed by Mario Testino.

What is it about Mr Testino? By a combination of charm, luck, hard work and patronage, he is now unassailable as the fashion industry's favourite snapper, the photographer du choix of every label, every magazine (though he's on contract to Vogue and Vanity Fair), every stylist and every model. He has managed to transcend the constraints of the frock shoot to become a sought-after portrait photographer, specifically requested by Madonna, Gwyneth, Angelina – and he was once, famously, called in by Diana for perhaps her warmest-ever sequence of images. Many admirers have testified to the "nonchalance" of Testino's photo-shoots, the relaxed, almost intimate, feel to the pictures, as though the overly-papped film and pop stars have discovered a friend, rather than a cold-eyed voyeur, behind the lens.

His skill is currently on display in the travelling exhibition of Vanity Fair Portraits, a gorgeous catalogue of famous faces and bodies from the magazine's two incarnations, from 1913-1936, and from 1983 to now. The photographers range from Man Ray and Cecil Beaton to Herb Ritts and Annie Leibovitz, and display a bewildering array of styles. Testino's work turns up all over the place – and the viewer is struck by how eclectic it is: saturated colour one moment, stern monochrome the next, close-up studio experiments, on-the-wing action shots. In the National Portrait Gallery's book of Mario Testino Portraits, a double-page spread of two of his works could be by wholly different photographers. One is of Madonna when she was filming Evita in 1996: black-eyed, black-furred, black-hatted, it accentuates the Material Girl's Mediterranean beauty in a composition as classic as a Sickert painting. By contrast, he photographs Jennifer Aniston in a horizontal flurry of soft-focus naked limbs, concealing her body from the camera but offering the viewer a teasing smile. Was he happier being classical or romantic?

He prefers the latter. He is almost dismissive about the Madonna shot. "This picture is probably the least her and the most a character she's playing. She was wearing brown contact lenses that completely changed her persona." And Jennifer? "With Jennifer, there's no role-playing. The picture is to do with a moment in her life, when she'd just split from Brad Pitt. I thought, let's make her sexy and attractive, and get hearts beating, you know?" Did he say, "Jennifer, I want to shoot you naked"? "Yes I did," says Testino, proudly. "I said, 'I want to show a side of you that's a little more daring, that's not the all-American good girl.'"

So that's how it's done. Another favourite is Gwyneth Paltrow, whom Testino has snapped in a thousand attitudes and styles, bringing out her demure elegance, her Vestal-virgin purity. The exhibition at the National Portrait Gallery highlights one extraordinary shot, white on blonde on white, that brings out her sad blue eyes; she resembles a reproachful seraph. "She's very poised," says Mario, "very clever, very cultivated, and I wanted to show that side, and bring out her softness." Did the lighting take ages to organise? He looks pained. "No – we were in the bathroom with just the light behind her. We were in this very grand house, it was the end of the afternoon. She was just going to try on a dress and I said, 'Wait, let's do it just here, now.'" There was, it seems, no technical wizardry, bar the Testino charm and the slight hint of double entendre in his urgent suggestion. "My photos are very simple. I believe the magic of photography is capturing the moment that doesn't exist a moment before or later."

How very Cartier-Bresson. Testino was born in October 1954 in Lima, Peru, the eldest of six children. His father was Italian, his mother was of Irish extraction. I wonder how his ideas of female beauty had been formed when young. "My mother was one of four sisters, very glamorous, well-groomed, well-dressed. But my real aesthetic comes from my own generation – the sexiness of the girls I was living with in my teens, the beginning of being carefree and wild. How they would take off their dress and there would be a bikini underneath, and we'd go for a swim and you could change in front of everyone without a problem." He was inspired "by my summers in Rio de Janeiro ... when I was 14, on holiday, and going from my house to the beach and seeing everyone walk everywhere in their bathing suits. I just could not believe it."

Testino's South American free spirit baulks at only one thing – male portrait subjects. When I ask why the men in his pictures tend to look ill at ease – Robbie Williams peeping from behind his sequinned Union Jack thong, Brad Pitt concealing his arms defensively behind his back – he is uncharacteristically censorious. "I have to say, when I photographed Robbie with Gisele Bündchen for the cover of British Vogue, he was naked the entire day, occasionally covering his private parts. I don't do men very often. I do mainly women, that's my line of business. Maybe a man in that situation, it's a bit... naff." Naff? What, to pose before a camera? "I'd much prefer a girl being sexy for the camera. A man should be more... discreet. You see a girl in a skirt with her legs in the air – but men shouldn't be showing their legs everywhere."

Mario, I say to him, what a prude you sound.

"I'm not a prude," he says. "But there's a billboard at the moment showing David Beckham in his underwear, with his balls hanging out. Personally, I don't find that attractive in a man."

We're on safer territory, discussing more of his favourite girls, such as Liz Hurley ("That's an unbelievable body, isn't it? Like a South American girl, not an English girl at all") whom he snapped by a pool in LA, posing heroically in shades and a mink coat: "I was trying to show that side of Hollywood, where a woman can walk round a pool in a bikini made of sequins and you know they'd never dream of going into the water." And he is happiest talking about Kate Moss, whose headlong career in fashion stardom echoes or shadows his own over the last decade. He adores "her taste, style, humour and kindness – she makes your day really worth it." In one extraordinary portrait, he seems to have painted her eyes and mouth with grungy clown make-up. Wasn't he taking liberties with the divine Kate? "It was she who did it," says Testino. "I put her in front of the camera, opened a box of make-up and said, 'Put your finger in the blue make-up and rub it on your eyelids. Then put your finger on the red one and rub it on your lips. She did it in front of the camera without a mirror." And the reason for doing it? "It was like saying, 'This girl's got it. You can put anything on her face and she still looks great.'"

Mario Testino never set out to be a portraitist. He just wanted to be a fashion photographer, snapping lovely frocks with some women inside them. "For years, people would say, 'Your pictures of clothes are beautiful,' but would never talk about the sitter. So I decided I had to concentrate on who I was photographing." His sympathetic attention renders models' faces beautiful because they are being treated as people rather than clothes horses; and that beauty effloresces into the arresting images now on display in the Portrait Gallery. It's a record of personal maturity that makes you warm to the Peruvian alchemist. Once you've stopped seething with envy, that is.

'Vanity Fair' Portraits: Photographs 1913-2008 runs until 26 May at the National Portrait Gallery, London WC2 (020-7306 0055); 'Mario Testino Portraits' will be reissued on 14 April by the National Portrait Gallery ( www.npg.org.uk), priced £40