Waiting to interview Fabien Constant, director of hotly-anticipated documentary Mademoiselle C, we overhear more than one person describing Carine Roitfeld, former editor extraordinaire and subject of the film, as the nicest person in fashion.

And it appears to be true as she greets us warmly at London’s May Fair hotel, laughing off mistaking our tentative ‘bonjour’ for fluency.

‘She brings such calm’, Constant says of the women he filmed for nine months as she created new magazine, CR Fashion Book. ‘She’s quiet and polite which means everyone can focus on work, not drama.’

Constant’s documentary goes behind the scenes of the fashion world, with famous faces including Lara Stone, Kanye West and Sarah Jessica Parker all making appearances.

‘Fashion is much more interesting than the superficial cliché, like clowns in the circus,’ he explains. ‘Look at Karl Lagerfeld; he’s one of the most interesting men in the world, his studio has books from floor to ceiling on every subject imaginable.’

The film captures a rare glimpse of the designer in family-mode. Pushing Roitfeld’s newborn granddaughter in her pram he asks her mother Julia if she’s nursing, quipping, ‘My mother married a milkman, so not to ruin her bosom’

‘Carine is obsessed with babies’ says Constant, and it shows; from her decision to put one on the inaugural cover of CR, to a scene where she tells Donatella Versace, 'I'm a grandmother, Julia, she has a baby. Can you imagine?!' ‘No’ comes the reply.

describes Roitfled as his ‘ideal woman’ in the film in what Constant calls ‘that sexy voice of his’, explaining, ‘It was a little difficult with Tom because he hates being on camera so it was a huge sign of trust in Carine to let us film him working’.

What has he learned from Roitfeld? ‘Nothing is free, you really have to work, even at Carine’s level. She’s on set at 7am talking ideas through because she’s obsessed with every detail. It’s artistic work but still demanding work.’

Roitfeld’s obsession shows in her iconic images, ‘She’s not interested in the girl-next-door or street style. She wants to dream’, Constant says. ‘That’s what fashion should be - not just selling handbags’.

Mademoiselle C is in cinemas on 20 September and on DVD from 28 October

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Mademoiselle C: 5-minute preview