In case you’ve missed it, Blake Lively is really into pantsuits these days. Like*, really* into pantsuits. Five-pantsuits-in-one-day into pantsuits. And for that, you have to thank her new film, A Simple Favor, and its costume designer, Renée Ehrlich Kalfus. In the film, Lively plays Emily, a foul-mouthed, martini-swilling fashionista who goes missing, leaving her son with her husband, played by Henry Golding, and newfound friend and mommy blogger, played by Anna Kendrick. To emphasize the strong will and mysterious nature of the character, Kalfus dressed Emily entirely in men’s wear. Here, the designer breaks down Lively’s striking new look, and reveals the styling secrets behind a great vintage suit.

When about the script was intriguing from a fashion perspective?

I’d been talking to [the director] Paul Feig, and we knew it had to be this stylish, fashion noir thriller. The conversation was that these girls should look just fantastic. We needed to make them so desirable and aspirational in a way. That’s what makes a movie fun.

Did Paul give any direction about what he envisioned they would look like?

In many ways, I think Paul is the kind of director—and I think he would say this himself—whose instinct and brain work in a way where he gets people around him who he trusts, and he really wants them to do their thing. We collaborated that way, and then, of course, the actors have their ideas, and through that collaborative mode, we make the characters. But early on, we knew we were going to bump it way up and make it very stylish. That’s why I was very excited. You don’t often get a contemporary movie where you can do that.

How much do you take into consideration the actresses who are playing the characters when you start pulling looks?

A lot. I often try to wait—I think about the character, and in many ways you can dissect a script by figuring out who these people are and what they do, and there are many outside forms to consider before whittling down to exactly who they are. So I try to stay open because obviously the actor informs the character, and then the costumes completely inform the character once the actors start trying them on.

What were on the mood boards for these characters?

For Anna’s character, it was Instagram and all the mommy bloggers, who are aplenty, and the way in which they try to represent themselves and get a following. There was a plethora of great research for that. With Blake, there was this whole confluence of “someone should dress like Paul [Feig],” and “I want to wear men’s suits.” Then these pantsuits came in, and it really came together for Blake’s character. It was tweaked as we collaborated in the fitting room for hours and hours.

Why did suits feel necessary to become such an important part of the movie?

When you see Blake being cast in this film, you think of Blake and her past and all those long legs and short skirts. In my mind, I thought, “The studio’s going to want her body; she’s so gorgeous.” Then she said men’s suiting, and Paul was totally in agreement. He instantly said yes. There was some play with, “Should it be feminized?” But we just went with powerful men’s suiting. Then we found in that completely covered look how sexy it was. Of course, there were a lot of references—Lauren Bacall, Katherine Hepburn, Marlene Dietrich—and on my mood board I had a lot of Helmut Newton photographs.

Blake Lively in A Simple Favor.

Emma Tucker

It almost feels like what Serena van der Woodsen would wear once she got a job.

That’s funny. I never thought about that, but it’s really funny. Nobody has said that yet. It’s very, very sophisticated. And it takes a very, very strong character to be completely in men’s suiting.

How much of an input did Blake have in her wardrobe?

We collaborated a lot. Through Blake and Paul both knowing Ralph Lauren, we were able to dig into the archives of Mr. Lauren’s men’s suiting. Those are vintage suits and archival, one-off pieces that we used. That was a gold mine. And not everybody can just put on a suit and walk out and look great, but she could really handle it, needless to say.

Everything is accessorized to the max.

The earrings were an idea that Emily had, all these piercings and this horn earring that was a piece of her past. The jewelry that she wears is all vintage men’s jewelry. It was found just combing through shops. More was better at that point. We kept saying, “Let’s put another one on.”

Was there anything that you found that didn’t make it into the film that you loved?

There was this amazing one-piece jumpsuit that was just divine. We ended up going for something else, and I don’t remember why. But I’m overall very happy with the choices that we made.

Did you know that Blake would continue to wear jumpsuits on the red carpet for the promo tour?

You know what, I’m so not surprised. She’s so savvy, and she’s so clever and so stylish. To promote like this is just brilliant. Often, funny enough, actors will leave and they will take a piece of that character with them. Blake is doing this just beautifully.