Warning: There's nothing below that's not mentioned in reviews and the trailer, but if you want to go in knowing nothing, read no further until you've seen 'Promising Young Woman.'

In an interview for VarietyCarey Mulligan describes the long-awaited "Promising Young Woman" as "a beautifully wrapped candy, and when you eat it, you realize it's poisonous." 

Or that's how the film's writer, producer and first-time director Emerald Fennell (a.k.a. Camilla Parker-Bowles in "The Crown" and the showrunner of "Killing Eve" season two) evocatively interprets her treatise of sexual assault and toxic masculinity fueled by men — and women. Earning raves (and a walkout) at Sundance in January, the story reveals itself to be more than a revenge thriller-meets-dark comedy as the plot unfolds. The costumes, by Nancy Steiner, also serve Mulligan's enigmatic Cassandra as camouflage, layers for the audience to try and figuratively unpeel.

"Emerald's vision for Cassie was to basically dress her the opposite of what you would think," says Steiner, on a call. 

The titular promising young woman dropped out of med school after best friend and classmate Nina's rape, and now spends her evenings trolling bars for vengeance, to expose and punish the hypocrisy of the "but I'm a nice guy" types. She carefully chooses her outfits to masquerade for her undercover missions and the parts she needs to play.

Cassie (Carey Mulligan) on a mission.

Cassie (Carey Mulligan) on a mission.

In the opening sequence at some danky happy hour spot (and set to "Boys" by Charli XCX, because this soundtrack!), corporate bros, including Jerry (Adam Brody, a.k.a, Seth from "The O.C.," in the first of many genius "nice guy" casting moments), down shots in requisite blue button-down shirts, ties and work pants. 

"We had an idea about the different bars around town and each one Cassie went to, she wore a different costume to fit in. So, [this is] the 'business bar,' where all the guys go to have a drink after work," Steiner, whose extensive resume also includes Sofia Coppola's "The Virgin Suicides" and "Lost in Translation," explains. 

Cassie feigns sloppy drunk, sprawled on a chaise in a black skirt suit, loosely unbuttoned white shirt and office pumps (above). "A hot fucking mess," one of the guys scoffs, while Jerry proclaims to chivalrously "check" on her — falling into Cassie's well-laid trap. After teaching him a lesson, Cassie gives "walk of shame" a new meaning as she strolls home, barefoot and with the shirt half-tucked. She carries her heels in one hand, while enjoying a gratifying breakfast sandwich held in the other. (Fellow sister-on-a-mission Harley Quinn from "Birds of Prey" would be proud.)

Club night Cassie.

Club night Cassie.

In the production notes, Fennell wrote: "One night she could be a beautiful, gorgeous hipster; one night she could be a woman who's just come from the office and had one too many; and one night she's in a classic body conscious, Kardashian dress. She's careful to be egalitarian when it comes to choosing her marks, I guess."

In her childhood bedroom, Cassie adds another hashmark and "Jerry" to her mini-notebook of patsies, filled with names like "Jez" and "Aaron." Now 30 years old and living back at home with concerned (and frustrated) parents, she spends her days working as a barista, alongside Gail (Laverne Cox), in a millennial pink-hued coffee shop complete with a neon sign reading "Make Me Coffee." (Pop-y '80s music videos and the sunny '90s "Sweet Valley High" television series inspired the also-vivid, confectionary-reminiscent production design by Michael Perry and set design by Rae Deslich.)

"She's this depressed woman who's stuck in her life, but she wears very feminine and sweet clothing, like it's all pink and pastel, and very light and airy," says Steiner." "You wouldn't necessarily expect that, which I like as it's a twist. Looking back on it, it feels like really in Cassie's real life, she was also wearing costumes."

Cassie at work handing Ryan (Bo Burnham) a suspect coffee.

Cassie at work handing Ryan (Bo Burnham) a suspect coffee.

Cassie carries her fuzzy, cozy pink robe and printed pajamas aesthetic to work with textured pastel floral sweaters and cutesy rainbow-printed raglan tops. Establishing another theme, the camera slow pans up to reveal Cassie's most ambiguous outfit (above): a rose-print, cap-sleeve bodysuit from Urban Outfitters, a white laser-cut belt cinching her slouchy jeans, a girlish yellow ribbon tied in her loose wispy braid and layers of heart chains and pendants. Her candy-color rainbow gel nails further enhance her intended misdirects.

"It's this feminine air that she puts on with the rose and floral print things. It's an armor in a way," explains Steiner. "So she's non-threatening. She just looks like a nice sweet girl. There's a cheery thing to a floral print and the pinks and the pastels. So nobody has to ask her questions, like if she's depressed or how she's feeling."

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In the shop, Cassie reconnects with an old med school classmate, Ryan (Bo Burnham), and agrees to go on a date with the seemingly pure nice guy. For the occasion, she continues her winsome floral theme with a romantic puff-sleeve dress, accessorized with heels and her signature gold necklaces.

"That rose print was actually designed by Emerald's sister, Coco, who has a clothing line called Coco Fennell," says Steiner. (The It Brit #cottagecore brand has been worn by Rita Ora and Rihanna, too.) "That was the most pronounced dress Cassie wore."

Originally, Steiner and Fennell envisioned a '60s-inspired Brigitte Bardot theme for Cassie's aesthetic, which evolved into a more contemporary translation of the blonde bombshell facade. At a certain point, Cassie lets down her walls, especially with Ryan, and her wardrobe starts to evolve.

"Even though the stars are blind... " 

"Even though the stars are blind... " 

In a singular blissful and sweet-as-candy moment, Ryan and Cassie run errands at a pharmacy, while performing their choreographed karaoke rendition of Paris Hilton's 2006 bop "Stars Are Blind." ("You would immediately love any man that knew every word of that song," Fennell told The Hollywood Reporter.) Cassie matches the mood in a vintage abstract floral top and highlighter pink button-up cardigan (above), which revisited the original '60s motif.

"The cute vintage '60s top and cardigan are less of a put-on — that feels more like her authentic self than anything to me and there's some character to it," says Steiner. "It's not just cute and sweet. It's just a little more saturated in colors. I liked that scene especially because they're doing the dance and singing the song, and it's so cute."

But Cassie's reprieve is short-lived and her avenger focus returns, culminating in her ultimate disguise: the quintessential sexy stripper outfit to infiltrate a bachelor party full of overgrown frat bros in polos, dad jeans and the like.

"Emerald specifically wanted men in 'kah-kees,' as she says it with her British accent," Steiner laughs. "She specifically wanted: 'men in khakis grinding and drinking beer.'"

Chris Lowell as the anti-Piz. "Now you do a shot, bruh!" 

Chris Lowell as the anti-Piz. "Now you do a shot, bruh!" 

Finding the archetypal douche-wear for the dudes — as also played by your IRL faves Chris Lowell (above) and Max Greenfield — proved straightforward, as did Cassie's ultimate disguise. 

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"I just trolled around online for 'sexy nurses' and came up with that [idea]," says Steiner, who custom built the costume (below) for multiples. "I'm not going to say it was some amazing design. It was really just a collaboration of how we wanted it to fit, where we wanted the hem — how long and how short — and putting in a zipper in the front, so she could play with it. And the little cross with the heart."

It'll only hurt a little bit.

It'll only hurt a little bit.

But there's a constant throughout the film that actually exposes the real Cassie and her paramount intention: one-half of a gold best friends/broken heart necklace, which has been hiding in plain sight.

"You just don't recognize it all the time. It's always there, maybe it's under something, you just don't know," Steiner notes. The viewer's realization of the custom-made piece coincides with the crescendo of the final climactic moments and the twists and reveals of Cassie's plans. Because all along the way, we never really knew her — and her disguises aided in the deception.

"This was a story where costumes do tell a lot about what's going on," says Steiner. "They always do, but basically Cassie uses costume to play her games and she is doing the dressing up. She's creating these characters. She's actually costume designing as a character in the movie."

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