Photo: Quentin Jones for Harper’s Bazaar I Carey Wears: Taffeta dress, cotton coat, both Prada

It’s not just any men – you recognise these guys, you’re fond of them. There’s Seth from The OC, McLovin from Superbad, Richard from Veep – all transformed into seemingly upstanding and yet reprehensible exploiters of a woman’s perceived vulnerability. Cassie, driven by grief for her friend, doesn’t stop there, but conducts a carefully plotted step-by-step campaign of revenge targeting each of her friend’s betrayers, from the lawyer who represented her rapist to the bystanders who turned a blind eye.

“When the script came to my agent,” says Mulligan, “I just didn’t know what to do with it. I thought, ‘Why would Emerald ask me to do this?'” The part, and the movie, were outside Mulligan’s usual territory and the challenge to do something different was irresistible. Mulligan is more often found in period pieces – The Great Gatsby, Suffragette, Far from the Madding Crowd. She tends to play characters more obviously sympathetic, such as the grieving widow, Edith Pretty, in her other new project, a Netflix film called The Dig, which takes place on the eve of World War II and tells the story of the discovery of a haul of Anglo-Saxon artefacts beneath Edith’s land. Edith is the quintessential Mulligan role – slightly pained, wry, and often giving the impression of a buried emotional life communicated through a tiny half-smile, or a flicker of an eyebrow.

Cassie, by contrast, is flamboyant and fierce. Mulligan delighted at the thought of playing someone who wasn’t meant to be ‘nice’, whose actions might make viewers feel uncomfortable. She did precisely the thing she’d promised her agent she would stop doing (“I’ve got in trouble in the past for committing to things out of enthusiasm”) and arranged a coffee with Fennell. They’d met, previously, at a friend’s house, when Mulligan had been struck by Fennell’s wit, and her excellent trousers. This time, though, it was straight to business. “I said, Em, I just have to tell you, I love it so much, please let’s do this,” she recalls. “And from then on, it was just super fun.”