Photo: Quentin Jones for Harper’s Bazaar I Carey wears: Grosgrain dress, wedge sliders, both Givenchy

Back in her former life, pre-children, Mulligan used to spend months preparing for a role. Before she played Daisy in The Great Gatsby, she went to Princeton and studied the letters between F Scott Fitzgerald and Ginevra King, the author’s inspiration for Daisy. The academic approach, she says now, was “all qualification:– I didn’t go to drama school, I kind of felt like a chancer, so I figured that I had to do loads of homework so that I was allowed to be here.” Currently, things are a little different, she says, laughing. “The reality of my life now is that I have two kids under the age of five, and I’m lucky if I can learn my lines and show up.”

Researching Cassie mostly took the form of conversations with Fennell. Together, they constructed her backstory and were inspired by work the director had done on the film’s look and feel. The finished product is a carnival of pastels and neons, Mulligan sporting big, blonde hair and giving off a dark, furious energy. Fennell had also made a Spotify playlist to guide the tone, including songs by Charlie XCX, Paris Hilton and two different versions of Britney Spears’ ‘Toxic’. Cassie’s wardrobe alone took some getting used to. Remembering one forbiddingly tiny outfit, Mulligan says: “I haven’t worn a dress like that in my personal life, probably ever.”

Photo: Quentin Jones for Harper’s Bazaar I Carey wears: Cotton trench-coat, Alexander McQueen

On the contrary: for years, Mulligan says, she would end up in tears on photo-shoots or after red carpets, finding the public performance of herself in clothes she’d never usually choose to be agonising. Now, she simply has a note that goes in advance to the stylist – no jeans, and nothing above the knee. “It’s so much easier,” she says. Having the confidence to make such a demand took a while to build – and speaks to a wider shift in her acting, too. No other actor I can think of has been so critical of their past performances as Mulligan. Often, in interviews, she would talk about what she’d got wrong. It has taken a while to channel that self-doubt into something more productive, a change she credits to Paul Dano, the American actor and director of Wildlife. During the filming of a scene when Mulligan’s character, Jeanette, is falling apart, she felt she repeatedly made mistakes. “I kept going, ‘Cut! Fuck! Sorry! Paul! Sorry!’ And he was like, ‘Stop cutting yourself! Stop trying to do it perfectly… just let her feel this stuff.'”