Vogue documentary tries to get a read on the chilly Wintour

Whether you like her or not, chance are you know who Anna Wintour is. Whether you want to sit through a documentary about her is another question.

Say you have no interest in fashion. Say you genuinely couldn't give a hoot about couture, handbags or whether this season's heels should be four inches or five. Say you don't even know what a heel is. There's still a smidgen of a chance that you've heard of Anna Wintour, the bob-haired, feared editor of American Vogue. Calling the 59-year-old British woman "legendary" is crass. Wintour, who has helmed the fashion tome since 1988, is a well-recognised name across the globe, consistently talked of as more influential within fashion circles than the designers. She can make collections, she can ruin them. The pieces that end up in her hallowed pages filter down and dictate trends for the fashion spectrum - from Gucci to Gap.

The extent of her influence is shortly to be revealed in a documentary on the Vogue office, released next month. Titled The September Issue, the film has been pulled together by the Emmy Award-winning filmmaker RJ Cutler, whose previous projects have included a documentary on Bill Clinton and, somewhat less exaltedly, a reality show titled Greatest American Dog. Filmed in 2007, the documentary examines the everyday life of the office as Voguettes scramble to put together the biggest issue of the year - 840 glossy pages, of which 727 are advertisements. The buzz surrounding its release is high-pitched, mostly because onlookers are salivating at the prospect of unveiling the enigmatic character.

Wintour has always been guarded. Cutler has said, in a nonplussed manner, that he simply had to ask her permission for the film, but in the past, interviews have been rare, and most have left the reader with no revelatory impression. Tellingly, Wintour once explained that she admired her father, Charles, a former editor of the London Evening Standard, because he was "inscrutable". It is a quality his daughter seems to have inherited, right down to the masking Chanel sunglasses that she so often wears. But can she really be so terrible? So terrifying?

Those who know Wintour say that her professional front belies a very different personal life. "Behind the publicly cold facade is a devoted and generous mother and friend; there is no limit to the lengths she will go to for those in that tight circle of people she trusts," insists one family friend, who prefers to remain unnamed. But that "publicly cold facade" has led to much speculation, and before now there have been notable attempts to unpick Wintour, both on-screen and off. In 2000, the BBC was allowed access to her for one episode of a series called Boss Woman. Viewers learnt that Wintour rises before 6am, plays tennis and gets to the office for 8am. She often eats steak for lunch. She rarely stays at any party for longer than 20 minutes and tends not to stay out late. But so far, so typical for a high-powered magazine editor.

In 2005, the biographer Jerry Oppenheimer published an unauthorised book on Wintour that was catty in tone but no great exposé. Often decried as "sizeist" (she once called the residents of Minnesota "little houses"), Oppenheimer wrote that Wintour used to buy clothes for a school friend that were too small, to try to make her feel fat. She has had the same haircut since she was 14, a topic that is given five pages in Oppenheimer's book. And once, in reply to a complaint from Cindy Crawford's agents about their client being dropped as a Vogue cover girl, she is said to have retorted: "Cindy Crawford's just another model. I'm Anna Wintour."

It was this haughty, ice-queen image of Wintour that was put to such brilliant effect in the hit film The Devil Wears Prada. Released in 2006, it made Wintour's name more mainstream, spreading from the fashion world into popular culture. Meryl Streep plays the withering Runway editor Miranda Priestly, given to outrageous demands. "Why isn't my coffee here? Has someone died?" she questions in one scene.

The film was based on a book penned by The New Yorker author Lauren Weisberger, who once briefly passed through the doors of Vogue as Wintour's assistant. Therefore, film sleuths insisted, Priestly's character must be based on Wintour, whose assistants talk of not being able to look their boss in the eye, of having to wear heels at all time in the office, of dancing through the office corridors when she is out of town. Wintour's reaction to the film, however, implied that she doesn't take herself as seriously as some suggest. She attended an advanced screening wearing Prada, and a spokesman subsequently said she found the film "very entertaining".

Perhaps The September Issue is her attempt to set the record straight before bowing out. Wintour turns 60 in November and since the end of last year has been dogged by whispers that she was to be replaced, possibly by the editor of French Vogue, Carine Roitfeld. Those chasing the story then met with ill luck. "I'm so sorry, I think that's an extremely rude question," she replied to one unfortunate reporter who asked about the rumour. "Just go away." Inscrutable as ever, it seems.

* Sophia Money-Coutts