The process of adaptation necessitates two early decisions: how true should I be to the original, and what shall I dump? There are times when murdering someone else's darlings is a more harrowing process than murdering one's own. Adapting for the opera stage probably involves more blood-shed than usual, but the nature of that transformation does require – and therefore offers – greater freedoms. The darlings appear to be asking for it.

Hardy called his story "An Imaginative Woman". There's less a shadow of veiled criticism in that, I'm sure, than an ironic admonition. Hardy's imaginative woman is assailed by a deep dissatisfaction with her life, with her husband and with herself. It's clear that her reach falls short of her grasp, and it's also clear that Hardy's sympathies lie with her.

Ella Marchmill is on holiday with her husband, William, and their three children at a seaside resort. William is a gunmaker. (We dislike him already, as Hardy intended.) The house where they're staying has two rooms reserved, says their landlady, for a young poet: "… a different sort of young man from most – dreamy, solitary, rather melancholy …" Having indulged himself with a caricature to play against the arms-dealer husband, Hardy then reveals that Ella is a poet manqué, that, writing under a male pseudonym, she has published a few poems, and that she has long admired the work of the man whom Hardy, with elbow-to-rib insistence, names Robert Trewe.

While William continues indifferent to his wife's nature, Ella falls in love with Trewe; with the idea of Trewe. She finds his coat and hat in a cupboard and puts them on. She takes a photo of him to bed. She writes to him using her pseudonym and makes several (failed) attempts to meet him. When news comes of Trewe's suicide, we learn that he left a note saying that the love of a good woman might have saved him. Ella's emotional wound is no less than that sort of blunderbuss irony might cause. Throughout the story, she has been pregnant with her fourth child. Very soon, she dies in childbirth; and, frankly, there is not much else left for her to do. Years later, in a flash of dark insight, William sees, in his youngest son, the image of Trewe and, believing him to be the poet's child, rejects him.

My apparent lack of generosity to Hardy here might owe something to having spent a good deal of time picking the bones out of this unashamed melodrama: deciding, in short, what to dump. As is most often the case, it was the composer – in this instance, Huw Watkins – who suggested our subject; and, in fact, it turned out a good suggestion: first because in opera subtlety is not everything, secondly because I could see that there was, quite clearly, something for me in Hardy's story, something I could shape to fit.

I changed the names of all the characters save Ella's. I did this, in part, because I simply wasn't comfortable with Hardy's choices, but also because I decided to give the story a contemporary setting; and in order, I suppose, to make the characters mine. The landlady plays a significant role in my version. Hardy says of her that she is "… a professional man's widow, left in needy circumstances by the sudden death of her husband". The words "needy" and "sudden" offered certain dramatic opportunities. Similarly, I wanted to develop the role of the husband, with whom I had some sympathy. And I wanted to bring the poet on stage.

I called him Ben Pascoe: an old pseudonym of mine. I suppose I was amusing myself, but for the adaptor, naming is a means to ownership. There are, in effect, two Pascoes on stage: Ella's passionate projection, and the man himself, who dislikes "poetry lovers" and is having an on-off affair with the landlady. Exchanges between Ella and Pascoe exist only in her imagination and rely, in part, on her reading of his poems – the lines go to and fro between them. In writing these passages, I used extracts from my own work. At first, I was startled to find so many that were a good fit; later I woke up to a deeper subtext: those mysteries of composition that are never evident while the piece is work-in-progress.

Ella's husband, in my version, is someone who makes money make money. Because such people now seem beneath contempt, he remained a reasonable paradigm for Hardy's William while, at the same time, providing me with the opportunity to frame a character whose role is more complex than merely oppositional. At one point, the husband, speaking of poetry, likens it to the language of high-finance, a kind of litany, rhythmic and highly-charged: "… like rollercoaster-swap or ankle-biter […] like bleeding edge, long jelly-roll, old uglies … / except with poetry it isn't money / it's love and sex and death …" He goes on to ask "…but what's it for?" (a question that poets often ask themselves).

Such a passage, and others like it, uses Hardy's story only as a reference. The same is true of the narrative line I've drawn in which Ella doesn't die, but allows her husband to believe that the child she's carrying is Pascoe's. It's a lie that frees her of her marriage. She empowers herself by means of deceit and the poet's suicide becomes, for her, a means to an end.

I've called Hardy's story melodramatic, and the broad strokes are there all right; but what first drew me to the story, and allowed me to see how it might be a fit for the opera stage, were the moments when Hardy shocks with sudden insight. Yes, his view of William is conveniently shallow, and his neurasthenic poet is stock-in-trade (though it's impossible to dismiss the idea that Hardy might have smiled as he wrote him). But his depiction of Ella's dilemma is distinctly proto-feminist. He speaks of "… the gloom of bearing children to a commonplace father", where "gloom", with its drawn-out mournful double syllable, puts the life she wants – possibly deserves – somehow beyond reach.

There are other, telling examples, and this not least: "… she went into the nursery and tried to let off her emotion by unnecessarily kissing the children, till she had a sudden sense of disgust at being reminded how plain-looking they were, like their father." When I was deciding what to lose from Hardy's narrative, first to go were the children: not least for practical reasons of casting. I think, now, that the opportunity to find complexity in Ella – but also, crucially, in her husband – and to develop them for the opera-stage, lay in that passage and others like it, where dramatic possibilities become apparent in what were almost asides.

• The Locked Room by Huw Watkins and David Harsent opens at the Edinburgh Festival on 30 August. It begins a tour of England and Wales at the Royal Opera House on 27 September