Gimme those cheesy rolls

Certain actors take the business of preparing for a role extremely seriously. For My Left Foot, Daniel Day-Lewis insisted on the crew pushing him around in a wheelchair between takes. Marlon Brando stuffed cotton-wool balls in his cheeks to play Vito Corleone in The Godfather, while Hilary Swank got headbutted in training for Million Dollar Baby.

Jill Scott prepared by eating an awful lot of cheese. 'We have this thing in Philadelphia, where I come from, called a cheesesteak,' she says, giggling. 'It's steak and melted cheese in a roll. I ate at least two of those a week. I've never been a small woman, but I put on 30lbs. Even then, they added padding to the hips, the heiney [bottom], the arms and the boobs.'

The role that required such carbohydrate-heavy method acting was that of Mma Ramotswe, the charmingly forthright heroine of Alexander McCall Smith's popular novel, The No 1 Ladies' Detective Agency. It is the first in a series of tales that chronicle the adventures of the only female-owned private investigation agency in Botswana and which became a worldwide bestseller when published 10 years ago.

The book has now been adapted for the BBC by award-winning director Anthony Minghella and Four Weddings scriptwriter Richard Curtis, whose themes normally centre around the residents of west London saying fuck a lot and enjoying mildly eccentric japes around hay bales. Scott is better known as a Grammy-award winning soul and R&B singer but gamely decamped to Botswana for a three-month shoot after a long audition process.

'I auditioned five times,' she says, her voice pouring like syrup down the transatlantic phone line. 'After the third audition, my mother was diagnosed with cerebral cancer, but she said, "Listen, you really want to do this." Then a few weeks later, the cancer disappeared. The doctor said it was like she never had it at all. I took it as a sign that if I got the role, I should really go ahead and do it and live life to the fullest.'

It is the sort of wildly optimistic plot twist that would delight McCall Smith, whose whimsy and unfashionably happy endings struck an unexpected chord with a reading public more used to misery memoirs and gritty post-modernism.

'It was so nice to play such a positive role,' says Scott, 35, who stars alongside Dreamgirls actress Anika Noni Rose as her secretary and Lucian Msamati as her suitor, the garage owner JLB Matekoni. 'It's such a rarity to do something that has no guns, no explosions. I mean, people died of old age in this film and that's lovely.'

It was Scott's first experience of Africa: 'I loved Botswana. I kept seeing myself in other women, bumping into myself and thinking, "Oh wow, there's my nose."'

McCall Smith also remarked on her likeness to the character. 'Sally - we all call him Sally - came on set a couple of times and he just looked at me and said, "Mma Ramotswe, it is so nice to meet you. You are exactly as I imagined you to be."'

It looks like the cheesesteaks paid off.

The No 1 Ladies Detective Agency will be broadcast on BBC1 over the Easter weekend