The search to find Precious Ramotswe, Botswana’s first lady detective, almost defeated me. I spent more than a year shuttling between London, southern Africa, Los Angeles and New York, hunting for someone with the necessary brains, beauty and bottom to inhabit the role.

Finally, it was with great pleasure, and not a little relief, that I witnessed the double transformation of Jill Scott. In front of the camera, she found a way to leave behind completely her true identity as a salty soul diva and arrive in the film with the traditional values and traditional build of Mma Ramotswe, the heroine of Alexander McCall Smith’s astoundingly popular series of novels, The No 1 Ladies Detective Agency.

The transformation away from the camera was equally profound. Scott is a singer and an American; not an actress, not African. Her first days in Botswana were almost catastrophic, the sky too big, the world too foreign. I thought she might go straight home to Philadelphia, overwhelmed by the size of the role, the stamina required, the absolute dislocation from what she knew. By the time we had finished shooting, however, she had planted herself firmly in the African soil and in the hearts of the Batswana women she was representing, surprising herself and all of us with her perfectly pitched performance. It turned out Scott had everything we needed. Only the bottom was padded.

Other than their clutch of wonderfully memorable characters, The No 1 Ladies Detective Agency novels are distinguished by their unabashed celebration of Africa in general and Botswana in particular. This was a powerful draw for my co-writer, Richard Curtis, and me, and for the producers, who came from southern Africa, the UK and the USA. It is arguable that most literature about the continent has been paternalistic, focused on what is wrong. Film has contributed pungent illustrations, documenting the tyrannies, diseases, disasters and violent upheavals that have convulsed Africa before, during and after the intervention of European colonisers. McCall Smith’s novels, in which Botswana is as much a central character as the lady detective, surprise a new reader with their constant assertion of what we might learn from the continent. They remind us of qualities we like to believe once existed in our own lives and countries - whether they did or not: a nostalgia for decency. As described within the pages of these books, Botswana is a serene place, communities exist, life is slow and peaceful, there is room for everyone, foreigners are welcome, happiness is possible.

It is rare for a director to be able to film in the location where a novel is set. Delighted to be able to shoot Botswana in Botswana, I couldn’t - at first - see how to conjure any of the key locations described in the books. Gaborone, the capital city, had none of the sleepy charm readers imagine when they picture Mma Ramotswe driving in her little white van from her delightful home in Zebra Drive to the detective agency, which nestles in the cool shade of Kgale Hill. As one local wag remarked to me: “I love the Botswana in those McCall Smith books, and would love to pay a visit.”

Yet, as I scratched away at this disappointing surface, another Botswana emerged, slightly different, perhaps, from the novels: modern, contradictory, but fascinating, a country on the hinge of past and future. Once, at dawn, I drove past the Game City shopping mall, a piece of London’s Brent Cross flung down in Gaborone, and counted 28 baboons scattered around the walkways, doing their own early-morning shopping, scouring the trash, grooming each other. Another morning, joining the traffic on a congested ring road, I witnessed a riderless horse galloping wildly down the dual carriageway. Cows, the centre of Botswana’s culture, are often to be seen crossing the street on some mysterious errand. I once bumped, almost literally, into a warthog. These encounters hinted at what was happening away from the main drag. If the Gaborone of McCall Smith’s books had surrendered to the inevitable erosions of progress, just beyond the city was a world more like the one familiar to his avid readers.

The climate was also a challenge. This is a dry, dry place. Clouds are an exotic sight. No surprise that “ pula”, the word for rain, is also the name of Botswana’s currency. I got used to the sensation of my sinuses being scraped with sandpaper. Away from the city, the parched earth crumbles and coats cars and humans in a clogging dust. As temperatures soared, even African actors suffered from heat exhaustion.

In a cold report, it’s hard to explain what it is about Botswana that so beguiled McCall Smith and the rest of us, me especially, who followed in his trail. Because we all fell in love, in our own way, so that, like McCall Smith’s American woman searching for her missing son in the second book of the series, Tears of the Giraffe: “I missed Botswana, and not a day went past, not a day, when I would not think about it. It was like an ache. I would have given anything to be able to walk out of my house and stand under a thorn tree or look up at that great white sky. Or hear African voices calling out to one another in the night. I even missed the October heat.”

The country has two of the natural wonders of the world: the Okavango delta and the Makgadikgadi salt pans. Yet, in the end, it’s the people who make you want to return, as many of us now will, to make a 13-part The No 1 Ladies Detective Agency series for the BBC and HBO. I am sure there are plenty of reprehensible Batswana. Otherwise, Mma Ramotswe would have no cases. Yet there is something in the culture that we would do well to study and borrow from. It is not just a matter of manners – although it never failed to touch me, the way of greeting, the left hand holding the right arm at the elbow to dignify the handshake, the little bob of dumela(hello), the courtesy. Somehow, this new democracy, however human and imperfect, has combined with powerful traditional values - respect for the old, respect for the young and an almost universal sense of patriotism - to create an enviable synthesis. “We had found a country where the people treated one another well, with respect, and where there are values other than the grab, grab, grab, which prevails back home” (Tears of the Giraffe).

An interesting phenomenon came to light in a recording studio where we had gathered together musicians to work on the film’s score: there seems to be no minor key in the country’s music. Many of the indigenous instruments can play only a major scale. This is certainly not true of all African music; a quick listen to the laments of Senegal will attest to the continent’s ability to mourn and find voice for its pain. Yet Botswana creates happy music and likes happy music. And the country’s tiny population has every right to sing. These are proud people, and an increasing number of them, enabled by their buoyant economy, the most successful in Africa, are well travelled and sophisticated, but so far this worldliness has not eroded their unfailing courtesy.

It has been said, a little inaccurately, that there are no minor keys in McCall Smith’s writing, either: “What we read contributes to the construction of our moral universe,” he once said. “I think that if we dwell on aggression, we shouldn’t be surprised when people turn to violence.” Most of the violence in his Mma Ramotswe stories is committed by animals, and what crimes there are tend to be solved over a cup of bush tea, rather than with a fist or a gun. It’s interesting that Mma Ramotswe is concerned less with the law than with how to be good. Again, this echoes something about Botswana’s own customary process, where the kgotla - the chief’s meeting place – serves as a forum for debating the behaviour of the village and still works in parallel with more familiar legal mechanisms. There is a saying in Setswana, “ Ntwa kgolo ke ya molomo”, which means something like “Peace can be achieved through dialogue rather than fighting”, and that philosophy runs deep in the culture. I know many Batswana will read this and think I am glossing their challenges and struggle, but I’ve been to enough places to recognise a good thing when I see it. And it seemed to me that Botswana is at peace with itself. That is a rare thing.

All filming experiences are indelible, stained as much by where a movie is shot by as any event recorded for the camera. The clear sense of being part of something good is what I carry away from Botswana and The No 1 Ladies Detective Agency. Perhaps this is as simple as being involved in a story about good people, about decency, or as banal as shooting a comedy after the murky emotional landscapes of my previous films. In any event, the experience was suffused with African light, with the primary colours of Botswana’s costumes and buildings and, to some extent, its people.

The No 1 Ladies Detective Agency is on BBC1 on Easter Sunday at 9pm

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