Phyllis Nagy talks about why the screenplay for Carol took her 15 years, her long friendship with the notoriously difficult Highsmith, and why the novelist would have loved Cate Blanchett in the leading role

It seems mean to praise a script for its silences. But Phyllis Nagy’s screenplay for Carol builds through a string of wordless exchanges. Carol (Cate Blanchett) and Therese (Rooney Mara) talk. But alongside the dialogue, a private, shared language of look and touch evolves, a whole glossary of silences.

Like Patricia Highsmith’s 1952 novel, The Price of Salt (later renamed Carol), on which the film is based, Nagy’s script is as much a story of the unsaid as the said. “Usually the first thing that is asked of you is to get rid of that stuff,” says Nagy. Storylines tend to be “overexplicated and expository”. But Nagy wanted this film to be “an immersive experience. Carol the novel is about observation and gesture. And so I was allowed to write scenes that had nothing in them but behaviour … That peculiar sense of sitting next to someone for the first time, smelling their perfume.”

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Perhaps that is why watching Carol is like stepping into a different world, and the difference simultaneously unnerves and excites. Carol and Therese’s relationship intensifies so incrementally, with such subtle signposting, that understanding it requires constant revision. Their love grows openly, though the film is set in New York in 1952. It seems at once improbable and unremarkable – and wilfully demanding of acceptance.

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Nagy, who is also a director and playwright, says that Todd Haynes, Carol’s director, “was sensitive and brilliant enough to let it be”. By which she means that the characters’ lesbianism “is not an issue. It’s not talked about. And when it is, it’s talked about as the state of normal. All the women” – including Carol’s friend Abby (played by Sarah Paulson) – “are lesbians. And I think it’s very potent. You’re just looking at any relationship.”

It was in 2000 that Nagy first began work on the screenplay. Her commitment to it has outlasted numerous, temporarily interested, directors including Stephen Frears and Kenneth Branagh. But maybe Carol is a better film for having been made to wait. “Fifteen years ago, we might have come across people who insisted on the gay speech or the guilt speech,” she says, an eyebrow lifting in horror. “It would have been the lesser for it.”

Facebook Twitter Pinterest Cate Blanchett as Carol. Photograph: Moviestore/Rex Shutterstock

Nagy likes “tricky books” – she is currently working on a Holocaust revenge thriller set in the world of chess – which is lucky, because Carol presents many challenges to adaptation. It is written in a close third-person. The narrator sits on the shoulder of Therese and makes regular advances into (and retreats from) her head. On paper, this creates a creepy sense of intrusion; Carol remains largely the property of Therese’s obsessive imagination. But how to translate this to film?

Nagy’s first decision was to split the point of view: the film begins with Therese and shifts to Carol. Crucially: “The point of view is always with the more vulnerable party.” She decided to make Therese a photographer rather than a set designer. This allowed Therese to be seen moving from objects to people. “Which is a bit like Pat herself,” Nagy says, referring to the author. “I mean, I knew her. And Therese is a clear stand-in for Pat Highsmith. It was very helpful in creating Therese, and the way that she might speak. Even the odd syntax Pat would use.”

Can she give an example?

Her voice sharpens and the line comes out prickly and needling. “A friend of mine told me I should be more interested in humans,” she says, looking up. “Like: who says that? Therese, who was flung out of space. And Pat, who was flung out of space.”

Highsmith herself was famously difficult, which must have created difficulties for Nagy, too. The two were friends. Was the author perched on Nagy’s shoulder as she wrote?

“I willed her not to, because she was terribly unhappy with every film that had been made of her books. She wasn’t alive for the Minghella Ripley,” Nagy says, pulling a face that suggests this is a good thing. Highsmith couldn’t stand Alfred Hitchcock’s Strangers on a Train; she found Plein Soleil ridiculous. “With our film, I like to think she would have finally thought we’ve got something that nails exactly what the book is about.

Facebook Twitter Pinterest Cate Blanchett as Carol and Rooney Mara as Therese in Carol. Photograph: Allstar/The Weinstein Company

“I think at the very least, she’d have thought” – and Nagy booms out Highsmith’s imagined reaction beefily, lasciviously: “‘Cate Blanchett. Yeah! That’s my Carol!’ So if she would bitch at me about anything, she would at least love the aesthetic of the thing.”

Nagy and Highsmith met in New York when Nagy was 22. She had been asked to accompany Highsmith to a cemetery for an article for the New York Times. “We had a largely silent limousine ride, and it was quite a long journey. I think she spoke to me three times out of the blue to interrogate me about certain American dramatists.” Nagy has this story off almost like a mini-screenplay. “So: ‘What do you think of O’Neill?’ I said: ‘No, not much.’ Nods. ‘Good.’ Silence, silence. ‘What do you think of Tennessee Williams?’ ‘Yes, I like him.’ ‘Good!’”

At the cemetery, they didn’t speak at all. Highsmith poked her stick at a few gravestones; then came “a gruesome tour” of the crematorium where they were invited to place their hands in the still-warm oven.

“We got outside. It was about 11am. Highsmith pulled a hip flask out of her trenchcoat and said: ‘I don’t know about you, but I need a drink.’” Then she invited Nagy back to her hotel for lunch, which, says Nagy: “Actually consisted of a minibar of beer.”

This was not, apparently, an attempt at seduction. Rather, Highsmith “used to collect – her word – young girls who were needing a mentor,” Nagy says. The two met up in London soon after and subsequently Highsmith wrote to Nagy “every single week. We had this huge correspondence. She was very sweet and encouraging to me as a young writer.”

Maybe the depth of their friendship helps to explain why Nagy stuck with the screenplay over 15 long years of non-production. Was it the presence of two gay female leads that made the film so hard to bring to screen?

“My feeling, having talked to and tried to put it together with so many people over the years, is not so much it being gay women; it’s about it being women. In film-financing terms, that’s very tricky. It’s very sad that I have to say this. But even Thelma & Louise was a generation ago. The 1930s and 40s were nothing but celebrations of fantastically complex female leads. We’ve lost that.” She mentions Bette Davis’s work with William Wyler, Sunset Boulevard … “You would never have that happen in a movie today, where a heroine inscribes a cigarette case ‘Mad about the boy’. She’d be called a bitch on Twitter.”

All too often, the central dramas or compulsions of female characters depend on men, Nagy says. “Women who are obsessed with getting pregnant, or jealous of another woman … This is the last acceptable punching bag, the woman. I think anyone in the industry would be surprised if we said there’s a fair amount of homophobia, because it’s the same subtle, or not so subtle, [prejudice] as with misogyny.

“If we’re talking specifically about gay women, about who they’re allowed to be, who gets to make the movies, it’s generally men.” She says she has noticed a swing back to “vampy” screen lesbians, “like Susan Sarandon and Catherine Deneuve à la The Hunger. You turn on the TV and there’s hot young dykes or bisexuals.”

But perhaps the strongest argument for bringing more complicated – and more gay – women to the screen is Carol itself. “If this movie does well,” says Nagy, “maybe three other movies like it can be made. And then if those movies do well …” Briefly, she allows herself to imagine a world in which the “state of normality” of lesbianism sells at the cinema in infinite variety and richness. Maybe it will happen. As a brilliant piece of art, Carol works. To change the world a little bit, it just needs to make money.

• Carol is released in the UK on 27 November.