After reading the script for Emerald Fennell’s revenge thriller “Promising Young Woman,” costume designer Nancy Steiner pictured a leading lady who was “disheveled” or, in gentler terms, “less-than put together.”

After all, Cassie — the film’s main character portrayed by Carey Mulligan — is depressed. The 30-year-old medical school dropout never recovered from a mysterious traumatic event, one that’s inspired her to fill her nights exacting revenge on toxic men. Yet in her daily life, Cassie is outfitted head-to-toe in bubblegum pinks and florals, with her long blonde hair often pulled back in a braid and decorated with ribbons. That’s all deliberate.

“Emerald really wanted Cassie to dress as if she was this happy-go-lucky girl. You would never know she’s depressed,” says Steiner, whose resume includes “Lost in Translation,” “Little Miss Sunshine” and “The Virgin Suicides.” “I’ve come to realize that it’s just another one of her costumes she wears to hide who she really is.”

Her girly exterior, what Steiner refers to as a “sweet all-American girl,” is part of Cassie’s chameleon-like tendency to hide in plain sight. Her everyday attire may be full of bold prints, bright hues and multicolored manicures, but rest assured, she’s not trying to stand out.

“I harkened back to the ’60s,” Steiner says. “I had Brigitte Bardot pictures on my mood board — a lot of cute blondes.”

As a costume designer, she views her job as “telling the story without shouting.” But, she says with a knowing wink to the movie’s shocking finale, “sometimes you need to shout.”

“Promising Young Woman” debuted in theaters on Christmas Day and released on-demand last weekend. Here, Steiner details key outfits worn by Mulligan in the movie.

Business Drunk

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Cassie at what the crew refers to as “the business bar.” Courtesy of Focus Features

Throughout the movie, Cassie frequents numerous clubs and bars to bait her prey. Behind-the-scenes, the crew gave each watering hole its own nickname to differentiate the vibes. “Promising Young Woman” opens with Cassie at the “business bar,” a traditional pub where men congregate after work. As she does in her everyday life, Cassie dresses to fit in. (At the “hipster bar,” she dons feather hair extensions.)

Steiner chose a traditional suit to keep Cassie the focus of the shot. Meanwhile, a montage of sweaty men wearing khakis and ill-fitting dress shirts plays over bumping music.

“We were trying to show how disgusting it can be to watch men from a woman’s point of view,” Steiner said. “You know, like, ‘Ew!’”

The Bachelorette

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“There’s a ‘Stepford Wife’ kind of thing about it,” Steiner says of Cassie’s date attire. Merie Weismiller Wallace; SMPSP

When she’s not tormenting evil men, Cassie moonlights as a barista. At the coffee shop, she reconnects with a former medical school classmate Ryan (Bo Burnham). After cute and bumbling banter, the two begin dating. One of Steiner’s favorite scenes transpires on a date, where Cassie and Ryan scour the aisles of a convenience store using snack items as karaoke props to belt the Paris Hilton pop anthem “Stars Are Blind.” She’s wearing a hot pink cardigan, a bright floral top, and blush-colored jeans.

“There’s a ‘Stepford Wife’ kind of thing about it,” Steiner says of Cassie’s date-night attire. “She puts on the pretty clothes and gets through the day. Nobody asks her about herself because she just blends right in.”

Impaired Eurovision

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Cassie at what the crew referred to as the “Eurotrash” bar. Merie Weismiller Wallace / Focus

Later, at what the crew dubbed the “Eurotrash bar,” Cassie dresses up in a tight, glittery sequined mini-dress, accessorized with ratty hair extensions, sky-high heels and silver hoop earrings. Cassie leaves to go home with her next pawn. But not before she awkwardly runs into Ryan while she’s draped over another man, who is wearing a fedora.

“She goes to these places thinking she won’t be recognized. I don’t know why Bo’s character was going to that bar,” Steiner postures. “I never understood that. It’s so not him. That was disturbing.”

Nurse Ratched

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“Promising Young Woman” costume designers wanted Cassie’s nurse outfit to be “sexy,” but not too over the top.

The movie comes to its climactic head as Cassie makes a surprise visit at the bachelor party of the man responsible for her trauma. A reference to their med school days, Cassie arrives at a remote cabin in the woods wearing a sultry nurse costume that’s complete with a multicolored wig and blood-red lipstick. Steiner says she couldn’t find the perfect costume, so she ended up making her own.

“We wanted something that wasn’t too over the top, but still sexy and taunting,” Steiner says. “It was fun to design because it was very different than Carey and Cassie.”