About eight or nine years ago I began to discuss a television adaptation of Pride and Prejudice with Sue Birtwistle, the producer. It was at a screening of Louis Marks’s lively, quirky BBC Screen 2 production of Northanger Abbey.

Pride and Prejudice, unsurprisingly, was Sue’s favourite Austen novel and mine too (I know it’s considered cooler to prefer Persuasion or Emma, but there you go). What we talked about in that first conversation is something that has got me into trouble subsequently - that at a profound level it’s a very sexy book, and that it would be wonderful to find a way of conveying that in an adaptation.

The other thing we talked about (perhaps it’s the same thing really) was the way that most Jane Austen adaptations seem to drown in social comedy and costumery - the men trapped in their buttons and stocks and jackets and boots, doomed to preen and bow and snicker in a succession of funereal drawing rooms, while the women blush and twitter and tremulously hope. We wanted to show that these people are not just stuffed costumes. We wanted to emphasise that Pride and Prejudice isn’t just a very funny social comedy, but also (and most importantly) a pretty desperate story about the power of desire and the power of money.

And it is, of course, just a matter of emphasis. All adaptations of P and P are going to have a great deal in common. I doubt if anyone would be tempted to mess about with the plot, which works like a Swiss watch, or the characters, in any fundamental way (though Aldous Huxley turned Lady Catherine into a secretly benevolent despot in his screenplay for the 1939 version). Neither Sue nor I wanted to do anything iconoclastic - this was our favourite book, after all. For example, we were determined to include as much of the original dialogue as possible, though not Austen’s authorial voice as such. Third person past tense voiceovers tend to have a genteelly embalming effect on the drama - see Scorsese’s Age of Innocence. (Though I was able to put a few favourite Austenisms into the dialogue. Lizzy and her father, as the principal ironists amongst the characters, got the best of them.) It wasn’t the first time I’d looked seriously at Pride and Prejudice. Like many of us, I’d read it first at school, for an exam, and liked it then, though not as much as I came to love it later. I had taught it, too, at Woodberry Down Comprehensive in London, and to trainee teachers at Warwick University, and I had re-read it simply for pleasure several times over the years.

Each re-reading of a great novel is different, and produces different insights and different emotional reactions. Reading with a screenplay in mind has some special aspects. Drama is about action. Character, at its deepest level, is what people do, not what they say, or even what the author says about them. And when we look at Pride and Prejudice like this, we notice that though it seems to be a book about Elizabeth Bennet and her sisters, they don’t start anything, and they don’t make things happen. The “inciting incident” (screenwriter’s technical term) is Bingley’s decision to rent Netherfield Hall. But the engine that drives the whole story is Darcy’s desire for Elizabeth.

This inconvenient (but ultimately rewarding) passion for his social inferior, begins (in our version) very early in their acquaintance (“I was in the middle before I knew I had begun”). We had to choose a moment, and in our adaptation Colin Firth takes another look at Jennifer Ehle within minutes of having made his disparaging remark about her at the Meryton hop. What is it about her that speaks to him so strongly? Her fine eyes - her quick darting intelligence, as apparent in her looks and glances and laughter as in her conversation. What else? She has enormous energy - she is always breaking into a run, as if impatient to get on with life.

“She looked quite wild.” Soon he is captivated by “the liveliness of her mind.” I think we’re talking about sexual vitality here, in sharp contrast to the deadly sophistication of Miss Bingley, and the lifelessness of poor (probably inbred) Lady Anne de Bourgh. And soon they are sparring with each other in a way familiar to us from Shakespeare comedies - it’s fascinating that these exchanges, even quite early on, have such depth and intimacy. I think that Elizabeth is strongly attracted sexually too, though she deeply dislikes Darcy, and her sexual feeling fuels her anger and her desire to hurt in the proposal scene.

A television adaptation gives us the opportunity (the temptation?) to see things a little more from Darcy’s point of view, as well as Elizabeth’s. And while as readers of the book we understand Darcy much better at the end of the book, the actor must understand him from his first appearance, and earlier. This led us to a slightly revisionist view of Darcy. A rather lonely childhood. Thrust early into responsibility by his father’s death. His close boyhood friend and companion turns out to be a scoundrel with a taste for underage girls, who nearly succeeds in debauching his sister. He (Darcy) blames himself severely for letting things get so far out of control. He’s a proud man. Lonely. Rather contemptuous of society. Formidably intelligent, but his feelings are so strong that they can overpower all rational considerations. Now, is that a Romantic hero or what?

Jane Austen famously denied herself any scenes which show men with other men, or men alone. When I began my screenplay, I decided to take the bit between the teeth and start with one of these - the moment when Bingley decides to rent Netherfield. Why? There were several reasons. Firstly, in a drama, important events should be dramatised, not reported as hearsay. Secondly, the desire to show these young men as physical beings, not tailor’s dummies. So they are riding big horses at a fast gallop across country, they are breathing hard, they are sweating, they are very much alive.

Also, that countryside is important - not for pretty-pretty reasons, but to show how huge and empty England was then, compared to now. And we see Netherfield Hall - it’s a splendid place, beyond our wildest dreams, and Bingley can take it on a whim - with Darcy’s nod. These are men of substance. Elizabeth Bennet, out walking, sees them from a distance - just two riders galloping - enjoys the sight, and then runs down hill to Longbourn, slowing as she nears the house (which we can all see is much more modest than Netherfield) and we can hear the sound of her youngest sisters quarrelling and her mother railing ineffectually at them, a sound with which we shall become increasingly familiar.

There was a personal reason too. I didn’t want to be too much in awe of a great original. It’s all right holding Jane Austen in reverence, but an obsequious crouch is not the right pose for an adaptor. I needed to say: “This is an interpretation, not a doomed attempt to reproduce the original” and I needed to say it as soon as possible, before I lost my nerve. The first invented lines of dialogue take a lot of chutzpah to produce, when the original is Jane Austen. (Not so much of a problem when it’s Michael Dobbs.) Later on, I was so much in the swing of things I knocked off Darcy’s proposal without batting an eyelid. (Look it up in the book. Austen only wrote the first few words of it in direct speech.) The first half of the book was a joy and a doddle to adapt (or so it seems in retrospect.) The second half presented formidable problems, because it has far fewer scenes in it: the characters go off in different directions and write each other letters. Darcy’s crucial long letter to Elizabeth is a case in point, but it offered me the opportunity to explore the Pemberley childhood and the Wickham back-story in flashback montage; and the section about Bingley and Jane enabled Elizabeth to re-experience in memory the full horror of her family’s conduct at the Netherfield ball. I also found myself unable to resist the temptation of following Lydia to her fate worse than death in the teeming metropolis (she takes to it like a duck to water, and Wickham looks a bit pale and worn out).

What’s it all about, finally? Well of course it’s a brilliant social comedy, and of course it’s a terrific love story, but I now believe it’s quite a Darwinian tale about natural selection. Darcy, as the Alpha Male, renews the blood line by going for energy, intelligence, sexual vitality - despite his “rational” objections, the Selfish Gene chooses correctly. Or, to adapt Blake slightly: the tigers of passion are wiser than the horses of instruction. Rather an unexpected moral for Jane Austen. And I hope people enjoy the adaptation as much as I have enjoyed working on it.

Pride and Prejudice will be broadcast in six parts on BBC1 starting on September 24 at 9pm.