When Francisco Costa succeeded Calvin Klein no one could have been more aware than he was of the scale of the challenge. But as this preview of his latest collection shows, he really needn't have worried. Maria Rose meets the designer in the brand's 40th year. Fashion by Charlie Harrington, photographs by Adrian Wilson

Francisco Costa's first foray into fashion was not exactly a success. He was 12 or 13, living in his native Brazil, when he was invited to visit his aunt and uncle for a week.

There was a rodeo and country fair on the agenda, and, remembers Costa, 'I decided I needed a special wardrobe.' So he sketched out a design and had a local seamstress whip it up. 'It was a burgundy wool-gaberdine safari suit,' he deadpans, and then bursts into laughter. 'Inspired by Saint Laurent! On the first day of the fair I got all dressed up, feeling so proud.

And then I realised my cousins, in their jeans and cowboy boots, were staring at me like I was a total freak. I can vividly remember the look of sympathy in my aunt's eyes. But I wasn't embarrassed. I thought I looked amazing.'

Luckily for Costa, his more recent work has been met with a very different sort of reception. As creative director of the Calvin Klein Collection for women - the brand's high-end catwalk line - he's won praise from critics and retailers alike for his clean-lined, sharply chic, eminently wearable clothes. Celebrities like Keira Knightley, Scarlett Johansson and Sienna Miller regularly don his frocks for the red carpet and this year he took home his second prestigious Womenswear Designer of the Year award from the Council of Fashion Designers of America.

Sitting in a conference room at Calvin Klein headquarters in New York's grungy Garment District, Costa looks as if he was born to work for the brand, which is celebrating its 40th anniversary this year. His crisp outfit (black shirt, white jeans) perfectly matches the room's minimalist black and white décor, and even his close-cropped hair (ebony with a few snowy strands) is in keeping with the corporate colour scheme. But, in reality, it's been only five years since the now 44-year-old Brazilian took over from Klein himself, upon his unexpected retirement two months before the winter 2003 collection was due to appear on the catwalk. Costa, who'd been working under Klein for just a year at the time, turned out a cohesive line that gave a glimpse of his potential. There have been a few missteps along the way - his spring 2007 collection was deemed tricky and derivative - but reviews have steadily improved and this year both collections have won raves, launching 'Francisco' into the rarefied fashion stratosphere inhabited by no-last-name-necessary legends such as Marc, Donna and Ralph.

It sounds like the trajectory of a man with very clear ambitions, but Costa describes his surprise promotion of 'taking Calvin's seat', as something he'd never imagined doing. In fact, he insists fashion is something he stumbled into.

Growing up in a small mountain town about three and half hours from Rio de Janeiro, Costa was surrounded by clothing from the start. His mother and aunt co-founded a childrenswear factory and he spent countless after-school hours observing the operation. 'It was very fun, very festive, very creative,' he remembers. 'But at the same time I was interested in so many other things. I painted. I drew. And when it came time to decide what to study, I chose accounting and business, not fashion.'

It was only after his mother died, when Costa was 22, that the thought of following in her footsteps entered his mind. Shortly afterwards he went to New York as a tourist, fell in love with the city and decided that he'd like to stay and go to college. He chose the Fashion Institute of Technology - the famed Manhattan design academy where Klein studied - and started taking night classes, devoting his days to improving his English.

Upon graduation, he scored an entry-level job with Susan Bennett Studio, a now defunct label. When his boss left to head up one of Bill Blass's secondary lines, he recruited Costa. From there Costa went to work on Oscar de la Renta Studio and then in de la Renta's own atelier, where he helped design ready-to-wear as well as the designer's couture collection for Balmain. Five years later Tom Ford hired him as senior designer at Gucci, and in 2002 Calvin Klein came calling.

In terms of aesthetic, it's hard to imagine three more different fashion houses than Oscar de la Renta, Calvin Klein and Gucci under Tom Ford's reign. De la Renta is known for turning out ladylike, ultra-feminine clothes beloved of New York socialites; Ford's designs exude unabashed sex appeal; and Klein is the master of sleek, clean lines. But Costa claims that adapting his point of view to suit these various labels never fazed him. 'It isn't about changing what I do, it's about evolving,' he says. 'It's a process of moving forward, learning and polishing.'

At Calvin Klein that evolution has mainly been about learning to hold back and pare down. Klein, after all, is fashion's Mr Clean, as famous for what his clothes lack (pattern, adornment, elaborate tailoring) as for what they boast (sharp lines, impeccable silhouettes and simple sensuality). 'When I first thought about coming to Calvin Klein, it was intimidating,' Costa admits. 'I thought, "How can someone be so precise, have such a strength and show so much restraint?" I never thought I'd be able to do that, but it's been fun learning to edit things down to what really matters.'

He didn't have much of an opportunity to learn from the man himself, but in the year between Costa's hire and Klein's retirement, the legendary designer made a huge impression. 'He was amazing, full of energy and truly curious,' recounts Costa. 'I remember the first time we selected fabric for a season. There were masses of samples, like I've never seen in my life. He considered them all one by one. I thought that was very telling, because you'd never expect someone involved in so many diverse areas to be so interested in the very basics.'

These days, Klein, 66, is no longer involved in the label, which he sold to the clothing conglomerate Phillips-Van Heusen in 2003, but his vision continues to provide the framework in which Costa designs. 'Calvin created a very strong visual language,' he says. 'My role here today is slightly different. I don't need to create that visual language; I need to maintain it.'

Still, it's not as if he's just rehashing Klein's greatest hits. 'It would be very mediocre for a designer to just go to the archives and recreate what was done,' he says. 'Calvin went though his own growth as a designer and I need to go through mine.'

For Costa a collection can develop out of any number of diverse inspirations. One season he might become obsessed with a particular fabric and use that as a jumping-off point. At other times it's a shape. 'I might see a vase and fall in love with it and think that I want to somehow achieve the feeling of that vase in a different way,' he says.

 The current collection, which is darkly glamorous and highly architectural, shot through with shine, started with an antique dress that Costa spotted in the window of a vintage shop while riding his bike. 'It's adjustable, almost like a puzzle, with all of the lines of the body very much defined and the measurements inscribed,' he says. The 2009 resort collection, available from December, began with a book on reptiles that Costa received as a gift. 'It has all of these amazing pictures of snakes,' he says, 'and then I happened to see a beautiful bronze sculpture of a snake while I was travelling in Europe.

Later I was looking at some Mapplethorpe photographs and saw one of a black man's leg in fishnet stockings. It had this beautiful shine, like the pictures in my reptile book. It's like, eureka! A light bulb goes off and it sets a mood.'

One constant is the influence of his homeland - a fact that will surprise many people. 'When people hear Brazil they think of the coast, where everything is fun and colourful - the spirit of carnival,' explains Costa, who tries to visit home about twice a year. 'Where I grew up, it's a very religious area with a sort of baroque atmosphere and people dressed much more simply, almost severe. Think of Sicily.'

For all his talk of inspiration and mood, however, Costa is, at his core, a realist. There are some fashion designers who view what they do as an art, one that shouldn't be sullied by talk of marketing and profit margins. Costa, refreshingly, is not one of those designers. When asked what criteria a dress or coat must meet to make it on to his catwalk, he doesn't hesitate. 'You have to look at it and be seduced by it,' he says. 'That's the only reason why we design new collections: because we want to sell. And what makes a sale? A piece of clothing has to seduce the buyer with great construction, great fit, great fabric and great style.'

And how does a fashion designer manage to pull off such seduction with heavy topics like war, the credit crunch and global warming so much on our minds? 'I think people will buy less, but buy better,' Costa says. 'So that's what I went for this season. There was a deliberate choice of stronger fabrics that suggest lifetime pieces, things you want to have for many years.'

But don't mistake the autumn collection's dark austerity as a reflection of gloomy times. 'Yes, we could get ourselves wrapped up in sorrow and sadness, but why?' he says. 'Fashion exists to excite.'
It's an attitude typical of the diminutive Brazilian, who is resolutely down to earth. Fashion, he insists, is 'just a job'. If Klein was Mr Clean, Costa is Mr Cool. 'I don't have any anxiety about the future,' he says with a smile. 'The next hour is plenty for me to think about. Let's just make it fun.'