Since 1979, the year that Imoved to London from Peru, it had been my ambition to become a fashion photographer. I had always liked fashion. I guess I acquired this sense of clothes from my mother—a great dresser—this sense of chic, proportions, and the right lengths.

At the time, I was drawn to the photographers David Bailey, Oliviero Toscani, and Chris von Wangenheim as well as to Richard Avedon, Irving Penn, and Helmut Newton. And I have been hugely influenced by Cecil Beaton. He represented everything that I didn't know when I went to England, and that I felt I needed to learn.

After nearly a decade of working in London, and a short-lived stint in New York, I began to spend more time in Paris. I realized that, much as I adored London, it was a great place to start, but if you wanted to become something other than the latest trend you had to go somewhere else. In Paris, I met the fashion stylist Carine Roitfeld (now editor in chief of French Vogue). We had the same sense of aesthetics. Our reference was the 70s—the hedonistic period when we were both young and loved dressing up. I started opening my eyes. Carine encouraged me to bring the personality of a personal project—for example, the black-and-white nude photographs that I had been taking using natural daylight—and apply that freedom to my fashion work. You've got to be adaptable with fashion. I started being noticed, and one day I got a call saying that Madonna wanted to meet me. She asked me to do an advertising campaign for Versace with her. I really had never thought of doing celebrities or personalities, because I was so obsessed with fashion and models and perfection. So when I photographed Madonna it was like a mixture, of fashion and portraiture that was a new type of fashion photography for me. At the time it was somehow very fresh. It's only now that celebrities are put on every cover of every magazine.

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Then I started working for a lot of the fashion houses and doing their campaigns. In the 90s, I joined forces with Tom Ford at Gucci, which was really the beginning of establishing my sort of photography. It was the period when I felt for the first time, This feels like a Mario Testino photograph, because for many years I had tried to emulate the English. I was so inspired and amazed by them and their style and their sense of reference and their Gainsboroughs and their Reynoldses! Now I wanted to give the girls a fresh vision, a fresh look—to make them look as beautiful as possible and of today. And that was the time, in September of 1996, when Meredith Etherington-Smith, who was then marketing director at Christie's, called, asking me to photograph Princess Diana to promote the New York sale of her dresses (which was Prince William's idea). The photographs were to run in Vanity Fair.

I SAID, "LET'S SIT AS IF WE'RE JUST TALKING, THE TWO OF US, ON THE SOFA," AND SO SHE LEANED BACK AND THAT'S HOW THE SHOOT STARTED.

At the time, they also asked Lord Snowdon to do the photographs of Diana for American Vogue, and Snowdon did the really classical royal photograph of her. I didn't have access to any palace or anything to make it look like that. I just had a studio, and I had one day to photograph her. So, in discussion with the stylist, Charlotte Pilcher, we decided that it would be a good idea to do Diana in a completely bare, simple way. Charlotte said, "You should do her with no jewels, without shoes—make her fresher and more modem rather than portraying her in the usual way."

We worked with the French hairdresser Marc Lopez and the makeup artist Tom Pecheux, my team for many years, and I said to them, "We have to make Diana look incredible and we should do a really natural feeling."

The actual day of photographing Diana, I was nervous at first because I didn't really know how to approach her. I first said to her, "It's really hard for me to call you 'ma'am' because I call my mother 'ma'am,' but she's a lot older than you!" So she said, "Just call me Diana." It might sound disrespectful, but for a photographer to get contact with somebody you need intimacy. In this case, just that name meant intimacy, and it created a huge difference, because it made me feel that this person was putting herself in my hands, trusting me a hundred percent.

I was quite amazed that she was so open about us doing anything we wanted to. When I introduced her to Tom and Marc she said to us, "I respect your work and I'll follow your opinion." Which was amazing because she'd created an iconic image of herself. When Diana arrived for the shoot she was dressed informally, wearing a shirt and a pair of trousers, but she had a "hairdo." I realized that she was quite "done up" for her age, but then, she had to fit into a role. From being a little girl she had had to become a woman. And what's a way of making somebody become a woman? Do a hairdo, you know. We just wanted to treat her like a young woman, in these evening dresses.

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I had been doing a lot of these photographs where the environment had become obsolete. After having been drawn to the funky, alternative world of London, which I couldn't have found in Lima, and which I had expressed in "out there" work for The Face and for Franca Sozzani at Per Lui and Lei in Italy, I later went into a more cinematographic world. I started telling stories. I used to produce all these very staged, complicated sittings, full of extras, and props, and sets. I remember Manolo Blahnik once saying to me, "My God, I love the floor in that house in Peru you photographed!" And I realized all of a sudden that the girl had become just one more thing in the photograph—a prop, like a table or a lamp! When I started working with Carine she hated props. She liked things simple, and I guess that made me focus on the girl, which is the approach we wanted to take with Diana too.

PHOTOGRAPHY IS ABOUT THE RIGHT MOMENT, AND I WAS LUCKY TO BE PRESENT AT THE RIGHT MOMENT IN HER LIFE.

So David Collins made a white sofa for me, and I just put some white panels behind it and used available light. I wanted just her, just Diana. It was about the dresses, but as all the dresses were different, they didn't have a real fashion connection between them. They were dresses she was letting go. She was entering a new period in her life. She had just been made Princess Diana, rather than Her Royal Highness Princess Diana.

We straightened her hair and did the simplest makeup. I think she felt she looked great. I said to her, "Would you kindly sit here?" on the white sofa. And she sat in a very proper way. Quite stiff. And I said to her, "You sit so properly, and if somebody told me to sit on that sofa, this is how I would sit." And I threw myself on the sofa! I guess that loungy, beachy lifestyle that I had for many years growing up in Peru helped in that instant, because she immediately burst out laughing. I said, "Let's sit as if we're just talking, the two of us, on the sofa," and so she leaned back and that's how the shoot started.

At the time, supermodels were the biggest, hottest thing around. I was going to all the fashion shows, and I could almost walk like each of the girls; they each have their own step, their own way of walking, like a dance. And if you're Peruvian you love to dance! I put on this CD of a disco version of the 70s French singer Dalida, and I said to Diana, "I'm really into the catwalk." She said, "What do you mean?" So I "did" each of the girls for her—Kate, Naomi, Claudia, Linda, and Nadja. Growing up in Peru, anything that was slightly effeminate was unacceptable. So for me it was liberating to be able to copy these girls—and I do it quite well! Diana couldn't believe it— she roared with laughter! I said, "Look, you can do it, too!" So we started doing the catwalk together—it was a really funny little moment. People become vulnerable in front of a camera and she had kept something back, but that broke the ice and helped her to relax.

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I had done work for Harper's Bazaar when it was being revamped by Liz Tilberis, and I learned a lot from the magazine's creative director, Fabien Baron. Fabien said to me, "Whenever you do a picture, the girl is really serious, but if one ever has dinner with you, one does not stop laughing." So I started evolving this idea that photography didn't need to be detached from me, that it could be related to my life and to what made me enjoy life. I started embracing the idea that people could have a good time in my pictures. I guess that is what I wanted to capture with Diana. An intimate moment. Because I realized that, although people the world over were obsessed with her, not many could find themselves next to her. So I thought, If I'm going to be that person next to her, let's try and get something that she would give only to me at that moment.

You try to do various things. So I photographed her on the sofa, and later I sat her on the floor. I've always liked the end of the evening, when girls are in their evening dresses but they've thrown their shoes off and they can sit or lie down on the floor. I wanted Diana dressed in eveningwear but in her mind to be wearing jeans.

At one moment I was dancing around, and I said, "Oh, I just want you to move and dance and laugh," and there I am moving and she was ... static! So I said, "Oh yes, but do you think it would be possible ..." and she said, "I know what you want, but I can't deliver; I'm not Peruvian!" It was a very endearing thing, because the English can be very reserved. And she was very proper. She was hugely proper. Not for nothing did she marry the future King of England. She asked me something that has always stayed with me: "When you were young, did you think you were going to have a big thing happen in your life? Because I always felt that something big was going to happen in my life."

WE SENT THE PICTURES TO DIANA, AND HER SONS TOLD HER THEY WERE THE MOST LIKE HER THEY HAD SEEN.

When I first edited the images, my friend Patrick Kinmonth, who's been my collaborator for the past 25 years, said to me, "You've chosen everything laughing." And he picked an image of her in repose; she was just soft and pensive. He said, "It's such a beautiful moment of her," and I realized that sometimes I dismiss those moments for the happy moments, because I know it is harder to get someone to laugh from the inside, real laughter. In some images I can see her insecurity—a sense of whether to just let go or not.

When we had edited the pictures and sent them to Diana for her approval, she said to me that her sons had told her they were the most like her that they had seen. This was a great compliment because, of course, I didn't know her. I just tried to grasp something on the spur of the moment. Photographers have to be intuitive about their work and less reflective and more proactive. I guess there are people that you meet in life, people that just have a very nice feeling to them, and you just hit it off with them. And that was the feeling I got from her— she was a sensitive, beautiful, kind person.

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People said, "You changed her," but I don't think I changed her that much, apart from the hair and a bit of the makeup, and maybe used the right light for her, but really I was more of a documenter who happened to be there in the right time and place. Later, I went back and looked at the clippings, the paparazzi photographs of Diana, and I thought, She does look different. All those photographs were taken of her in a very detached way by people who managed to get a glimpse of her at a certain distance. But what we were doing was obviously a much more intimate experience. It was influenced by the fact that I liked her, so I wanted to make her have a good time, make her laugh, make her enjoy that day. And I wanted the results to be worthwhile for her cause, to raise funds to donate to people with AIDS and cancer, two diseases that have been very close to me. Cancer through my family, AIDS through my friends.

The reaction after the pictures came out was pretty amazing. The "Diana" Vanity Fair became one of the top-selling issues in the magazine's history. I never imagined the shoot would have the repercussions that it did. It was odd, really; you've slaved for 15 years to get your pictures right and suddenly the world knows you for this one thing.

Diana died two months after the pictures were published. I never think of her any other way than how she looked in them.

I guess photography is so much about the right moment, and I was lucky to be present at the right moment in her life. I think I am so privileged to have been able to document her like that. She was really divine that day. She looked so happy and fresh and sure of herself. It was just laughter and laughter and laughter and laughter and laughter.

Excerpted from Diana, Princess of Wales by Mario Testino at Kensington Palace, to be published this month by Taschen TBC; © 2005 by the author. An exhibition of the same name will open at Kensington Palace on November 24.