FILM / 'I want to say, this isn't the way it is': The novelist and screenwriter Anita Desai describes the film's (In Custody) genesis

London, Cambridge, New Delhi, Massachusetts: meet Anita Desai, jetsetter with a twist. She rents a house in Cambridge, USA, where she teaches creative writing at the Institute of Technology. She has a cottage in Cambridge, UK, where she's a visiting fellow at Girton College. She's a member of the Advisory Board for English at the National Academy of Letters in Delhi, where she has a flat; a fellow of the Royal Society of Literature in London, where she stays with a friend in the grandest part of Holland Park. And she spent last summer in Cornwall.

It's an alarming itinerary, matched only by an alarming reputation. Desai is known for her sharp intelligence. Her novels and short stories, which she writes with enormous difficulty - 'agony, actually' - when at home in New Delhi, pull tight threads around human pretension, bristle with frustration at the hurdles of Indian life and balance ambiguities with a cool head. Merchant Ivory have been trying to convince her to write a screenplay for them for years, but she only succumbed when Ismail Merchant read her 1984 Booker-nominated novel, In Custody, and told her 'This is the book I want to film.' ('He understood the book very well; it's his world, it's his India.') The result, his first major work as a director, is a wash of blues and greens and sunburnt stone. 'Glorious technicolour,' she says in a tone of quiet, utter contempt.

Anita Desai sits upright and almost motionless, a neat woman in trim clothes, with fine features, hands resting still on her lap or against her face, and a voice so soft that transcribing the tape of the interview became an acrobatic feat, adjusting the volume between booming questions and answers so muted they melt into the whir of the machine. But every word is meticulously placed, which is what you would expect from someone whose life has been spent steering round the pitfalls of language.

Born in 1937, she is the daughter of a German mother and a Bengali father; she was brought up in Old Delhi but spoke German at home and English at school. Her books describe daily Indian life, but are written in English, her first language, and sell more copies here than anywhere else ('the state of translation in India is a sad one'). Americans find her culture 'very alien' and her split life turns her into two people: 'When I arrive in India, I put on cotton saris and find myself changing gears. In Delhi, I am part of a family, part of a community. In America, they don't know me at all. I'm just an individual.'

In Custody, about the slow erosion in India since Partition of the Muslim-spoken Urdu, stems from her childhood. 'I grew up at a time when there was still a Muslim presence,' she explains. 'So many people I knew in the neighbourhood were Muslims, and spoke Urdu - poetry was constantly being recited - and then in '47 they just ceased to have that presence. It always seems to me since like a ghost - still existing, but as a ghost.' Merchant has filmed the book in Urdu, which pleases her. But she admits, nicely, humbly, that it's about the only thing that does.

'When you're making a film, I've learnt, you have to be very overt, very blatant and you can't allow those ambiguities which interest me as a writer. So, for the sake of the film, we had to alter many things. For instance, the women in the book don't play a part, they're peripheral, and I meant that as a comment on the society. Women are peripheral, they're left out and shut out, but that wouldn't work in the film . . .

'It's very disconcerting because in your own mind you visualise it in a certain way and then you see it transformed by someone else's vision. I want to question it and say, 'How could you?' and 'This isn't the way it is', but of course it is - it's how he sees it. I realise that it would be completely wrong to hold on to it and make it the way I wanted it to be, it would be lifeless. As it is, a life of its own has been set free.'

But how would she have done it? She laughs very softly. 'I would probably have set it in a Delhi slum,' she says, 'and filmed it in black and white.'