Blake Lively’s engagement ring in A Simple Favor was inspired by Princess Diana’s

Warning: Minor spoilers ahead. 

From crystal-studded Christian Louboutins to a snap-off dickie (yes, dickie), there is more than a concession stand’s worth of fashion eye candy in the dark comedic thriller A Simple Favor. But one accessory truly stole the spotlight: the giant sapphire-and-diamond engagement ring that Blake Lively’s character, Emily Nelson, wears, which once belonged to the character’s mother-in-law — and serves as an important plot point for the film.

The dazzling piece bears an uncanny resemblance to another (very famous) family heirloom, and according to the film’s costume designer, Renée Ehrlich Kalfus, that similarity was on purpose.

It’s a take on Princess Diana’s ring,” she says, referencing the 12-carat oval sapphire surrounded by diamonds that’s now worn by Kate Middleton. “I wanted to do something vintage, but the director [Paul Feig] thought it should be more on the nose.

The traditional ring is an interesting choice for the anything-but-traditional Emily. She’s a high-powered fashion publicist who works in New York City, but lives in a Connecticut suburb with her husband and son, much to her dissatisfaction. “She doesn’t care about fitting in,” Kalfus says of the character. “She had her own agenda.

To convey that message, Kalfus looked to a trio of iconic Hollywood stars — Katherine Hepburn, Lauren Bacall, Diane Keaton — as well as the movie’s director for inspiration. “Paul goes to work every day in a Savile Row suit,” says Kalfus of Feig, who also directed Bridesmaids and Spy. “He feels he can get anyone to answer anything while wearing a suit.

She then tapped Feig and Lively’s contacts at Ralph Lauren for access to their vintage archives and was able to use several looks, including a three-piece suit and a killer tuxedo, all custom-tailored for Lively. Kalfus decked out each look with men’s jewelry like vintage watch fobs and multiple tie bars. “It gave her more armor, more stuff to keep people away,” says Kalfus.

Even covered up head-to-toe in a suit, Emily finds ways to flash more than just a little skin. In one early scene, she removes her jacket and shirt (which turns out to be not much more than a collar), before enjoying a gin martini (her signature drink) in a low-cut vest and a pair of faux cuffs, while her new (and visibly flustered) mom friend, Stephanie (Anna Kendrick) looks on.

I don’t think we told Anna that it was going to happen,” says Kalfus of the striptease-like sequence. “It was just a way of making Emily more intimidating and kind of weird.

In a later scene, Emily appears to forgo a top altogether under her suit jacket, although Kalfus swears she had on a blouse. (“It was very strategically tucked into a cummerbund, just enough to give a little side boob.”)

Accessories added another element of sex appeal, most notably Emily’s enviable collection of Louboutins, which we get a glimpse of in a shot of her enormous closet. “Because Blake has a relationship with Louboutin, we got boxes and boxes of boots, loafers, and heels.” All of the footwear is spectacular, but one pair of six-inch sparkly ombre Louboutins  stands out to Kalfus. “Blake had to run in them and she was like, ‘It’s fine, I can do this.’”

Not to be completely overshadowed by her glamorous friend, Kendrick’s Stephanie — a DIY mommy vlogger who’s desperate to fit in — has some memorable style moments of her own in the movie, including a hilarious scene featuring a pair of cat socks from Target.

After spending more time with Emily, Stephanie gets inspired to up her own style game by wearing an Alice + Olivia pom pom sweater and floral skirt (below). “When Anna put on that outfit she said, ‘I’m not sure I get this,’” says Kalfus with a laugh. “And I said, ‘trust me.’”

Kalfus pushed the costume boundaries with both characters for a reason — to suck you into the movie. “You should want to be one of of these women while you’re watching it. It’s that aspirational piece that gets you involved in the story.

People