Inside the 'Promising Young Woman' Music, From a Paris Hilton Sing-Along to String-Quartet Britney Spears

Merie Weismiller Wallace/Focus Features

Carey Mulligan and Bo Burnham in "Promising Young Woman"

Writer/director Emerald Fennell & composer Anthony Willis walk us through the many musical highlights from the buzzy Christmas Day release.

It all started with a playlist.

That playlist led to Promising Young Woman, one of the buzziest movies of the year (out in theaters Christmas Day), with its pop-packed, female-forward soundtrack and a score that seamlessly navigates between comedy and romance, tragedy and triumph, all with a dusting of horror for good measure. For her feature directorial debut, in which Carey Mulligan's Cassie is out for revenge after a traumatic college experience forever alters the course of her life, Emerald Fennell let music take the lead.

"Anything that I'm writing, I'll make a playlist and I'll listen to it over and over and over and over again till it's almost like it's sort of engrained in my brain," the onetime Killing Eve showrunner and actress behind Camilla on The Crown tells Billboard. "And then I'll write the movie in my head over the course of a couple of years, really, and the music usually just guides that process so much, and by the time I'm ready to write it down, it's sort of finished."

To make some of the dream tracks on that playlist a reality (including Charli XCX's "Boys" and Paris Hilton's "Stars Are Blind" -- more on that one below), Fennell partnered with Capitol Records, which also brought brand-new songs by Cyn, Fletcher and more young female artists to the table. "Not only did we end up with the stuff that I really wanted, those kind of more iconic moments, but we then had songs just tailor-made for the film," Fennell says. "That's what I loved about soundtracks when I was growing up, was new songs that made me think specifically of the movie. Anton Monsted -- who worked at Capitol and who's just incredible and who helped shape the soundtrack -- his first job was Baz Luhrmann's Romeo + Juliet [in 1996], which was my ultimate obsession, forever obsession. It was about doing something like that, where I hoped that in the future, young girls like me, when I was listening to the soundtracks, kind of listen to it and make up their own stories. That's what I felt was possible with them."

While the soundtrack was falling into place, Fennell dreamed of adding a "Hitchcockian, quite old-fashioned" score to the film as well, enlisting composer Anthony Willis -- who happened be an old classmate and remembers watching Fennell in school plays -- to see if he could bridge that gap. "Emerald very sweetly sent me the film to watch it," Willis recalls. "And I think really she, in the back of her mind, was thinking, 'Anthony's got a lot of experience on very thematic scores. I know composers who might do an interesting quirky score, but if we have him watch the film and write some music, will he be able to find a thematic hook for Cassie's journey?'"

Willis zeroed in on the core relationship between Mulligan's Cassie and her college friend Nina to ground the score and tie together the film's many tones. "What struck me the most when I first watched it was how brilliantly [Mulligan] plays the role of this woman who's really in this kind of purgatory," Willis says. "She's stopped living her own life back in college ... and as I started writing the theme, what I realized was it wasn't really her theme; it was a theme for a lost friendship."

Fennell acknowledges that the film's pre-existing pop soundtrack "was a blessing and curse for Anthony scoring it, because it needed to find its own very specific place. And I think, luckily, Anthony is just brilliant melodically, but also I knew I wanted a kind of orchestral score, rather than what I think would be maybe a more obvious choice, which is kind of more electro, more synthy. It sort of stabilizes it, Anthony's score. He was so wise, because everything's based on a theme and it's called 'Nina's Theme.' Nina feels very present in the movie because of that score and because it's in every single moment."

Another thread linking the score and soundtrack is Willis' unsettling string arrangement of Britney Spears' "Toxic," which sets the tone for a thrilling final act that we definitely won't spoil for you here. Below, Fennell and Willis discuss that key "Toxic" reimagining, plus Fennell leads us through some other major music moments in the film, from finally giving "Stars Are Blind" the shine it deserves, to really, truly listening to the lyrics of "It's Raining Men."

Anthony Willis' string arrangement of Britney Spears' "Toxic"

Anthony Willis: That was entirely Emerald's discovery. She had this idea like, "I want to take the idea of doing Britney and make it a super dark version of that." Which is so clever.

Emerald Fennell: We tried lots of instrumental versions there, and they weren't really working. And I sort of felt like, since I'd written the movie, that had been done. It didn't feel invigorating anymore. I found a kind of amateur string quartet on YouTube playing it, so it wasn't perfect, and then just slowed it down. And so then what Anthony did brilliantly -- and I sent it to him and I was like, "Am I crazy?" He's like, "No, I think we're on to something here" -- so then he rearranged everything and composed a piece that had everything from "Toxic" in it. And then we just asked the Viennese string quartet to play it like they were drunk. [Laughs] And then we slowed it down again, so poor them -- the best musicians in the world and I was like, "Can you play like you're drunk? Thank you!" [Laughs] But the thing about slowing it down is it's not entirely pleasant to listen to. It's not sort of neat and tidy. It's very physical and visceral.

Willis: There were so many songs in the movie that are used really ironically in the settings they're used in, but "Toxic" is different because it's not a pop cover; it's more of a score, thriller-based cover. "Toxic" is one of the greatest pop songs of the [2000s]. It's such a fantastic track and has such an evocative lyric. And so if you hear that riff and you hear that tune, you're imagining the word "toxic," but you're not actually hearing it. It was very much Emerald's idea to do that. What we did is I did a string-quartet arrangement of it and we slowed it down and walked it and we added some tuning up from the full orchestra that we recorded the score with. It's such a cool turning point in the film and it's really cool to see people enjoying that moment in the film.

Fennell: We needed something that was a bridge between the score and the soundtrack, because both are a huge part of the movie, but they're very distinct. So there needed to be a moment that felt like it was bringing them together. "Toxic" is one of those songs that just kind of hits you in a very specific place. And what's so wonderful about it is it's so famous, it's so well-known that you know the lyrics; you don't need to hear them. So it then gives you that relationship where you're bringing your own kind of knowledge to it.

Willis: I really see "Toxic" as that moment where all the kind of musical components of the film come together. You've got a really classic pop song done in a very horrific, string, horror/thriller style. So it's that bridge where they both meet. Doing that on strings definitely helped us with the sort of string, thriller, Hitchcock angle, for sure.

Paris Hilton's "Stars Are Blind"

In one of the comedic highlights of the film, Bo Burnham's Ryan begins singing along to Paris Hilton's "Stars Are Blind" in a drug store, much to Cassie's amusement and embarrassment. But midway through the 2006 pop song -- a top 20 hit for the TV star/heiress on the Billboard Hot 100 -- Cassie is also singing every word.

Fennell: I just love that song. I think it's so brilliant, so part of it for me was having a scene where it had to be kind of a classic rom-com moment, where two people sort of fall in love, and I had to think of what song, if a man knew every word to it, would make me fall in love with him -- and it definitely wasn't, like, the Rolling Stones deep cuts or like some very esoteric French 1920s LP. It was Paris Hilton's "Stars Are Blind." Because that guy is the guy that I want to get to know.

Has Paris seen the film yet?

Fennell: I don't know that she has. I met her very briefly and thanked her. I wrote to her because we needed the rights to it before filming because they're singing along to it. So I wrote to her and said everything that I just said to you, which is that I love it and it was a really important moment in the movie. And she let us have it. I bumped into her weirdly at a sort of work thing, and she was so gracious and so wonderful and so supportive, and I think, like Britney Spears, like a lot of female pop artists and celebrities, they've been used to being sort of misrepresented and taken advantage of, all of the things that these women have done, so I think it's important to treat their music in the way that it deserves to be treated and that I think it's brilliant and it's incredibly moving and it has a profound effect on people who listen to it, because you're bringing your own relationship to something to the film. It's one of my absolute favorite songs. Even now, if ever it comes on, on my shuffle, I will listen to it the whole way through.

DeathbyRomy's cover of The Weather Girls' "It's Raining Men"

Fennell: It's just such a great song. It's all the things that I wanted to have in this movie, which is very female, specifically female. It's very much karaoke, 2 a.m. at your hen party, your bachelorette party. But also, the lyrics, when you think about them for more than five seconds, are horrific. [Laughs] Because if the world was raining men, it would just be bodies hitting the floor! It would be a bloodbath. And that just seemed so wickedly funny.

"Something Wonderful" from The King & I

Fennell: That is one of the most romantic songs of all time, but when you listen to it, it's about a man who's incredibly cruel and callous, but once in a while, he does one good thing. He does something wonderful. That's what that song is about!

Juice Newton's "Angel of the Morning"

Fennell: That was the toughest one, actually. Because we had so many things in there. And that was brilliant Jenny [Swiatowy], from Capitol, who is just a genius. For ages, it was "I've Had the Time of My Life," which, it was so difficult, because for the last moment of the movie, we needed something that probably distilled the whole thing -- without being spoilery -- which is funny and moving and sad and warm and all the different things and also had lyrics that could be sort of twisted. "I've Had the Time of My Life" was funny and it was kind of anthemic and kind of air-punchy, but it also undercut the emotion a little bit. And, you know, it was slightly too far for me to steal from one of the greatest films of all time [Dirty Dancing]. We tried everything. We tried, I think, every single song that existed in there. And then Jenny said, "What about 'Angel of the Morning'?" And it was just like, "Oh, of course, that's it. That's the one."

Promising Young Woman hits theaters Christmas Day. The soundtrack and score are available for purchase and on streaming now.