hen the opening notes of “Paris” sound off, the crowd at Forest Hills Stadium in Queens, New York knows every word. The Chainsmokers — real names: Drew Taggart and Alex Pall — penned the escapist earworm months before around 4 a.m., after wrapping up a show in Europe and throwing back a few drinks: "We were staying in Paris / To get away from your parents."

The VMA nominees may be DJs by trade, but their outdoor performance in New York feels like a full-blown music festival, not a nightclub. The drummer and keyboardist they found on YouTube are just as important as their DJ booth. During “Paris," all eyes are on lead vocalist Taggart — except he isn't the only person singing.

You can't miss the dreamy, anonymous female voice that joins him for the chorus. Who's the mystery girl?

Enter Emily Warren, a Grammy-winning songwriter who's crafted many of the hits you can't get out of your head: Charli XCX's "Boys," Dua Lipa's "New Rules," and Fifth Harmony's "Them Girls Be Like," to name a few. Warren also co-wrote a good chunk of The Chainsmokers' debut album, Memories...Do Not Open. At the last minute, Taggart asked her to back him up vocally on "Paris," and she was happy to help.

"It's almost better than being featured because it caused such chaos," Warren, 24, told me about her uncredited part. She sang on or co-wrote four more tracks on Memories, so when the guys hit the road last April, she was tapped to join them. Pall called her the tour's "missing puzzle piece."

"The one side of writing with Emily, especially if she isn't going to be singing the song, is then trying to find someone that sounds better than her," Pall wrote in an email. "That's a real challenge."

Warren spoke about The Chainsmokers in equally glowing terms when I spoke to her over breakfast in between tour stops. She's worked with Taggart and Pall since 2015, so she's witnessed their gradual evolution from scrappy upstarts into bona fide pop stars. Watching their careers prepared her for her own solo debut, but before we get to that, let's leave "Paris" and travel to New York City — Warren's hometown — where her story begins.

Emily Warren & The Betters

By the time Warren arrived at New York University in 2011, she'd already made her musical mark on Manhattan. She grew up playing piano, guitar, and harp, then joined a band, Emily Warren & The Betters, all before she could legally consume alcohol at the venues where she performed.

But during her first week at college, The Betters called it quits, a breakup Warren still feels responsible for. She was younger than her bandmates and unsure about the commitment. "They were, like, quitting their jobs to be in this band," she said, "and I was like, whoa, that feels like a lot. I don't know what I'm doing yet."

The split led Warren to consider songwriting, though she hesitated initially. Her lyrics felt like a "diary entry," so how could she give them away? Fellow songwriter Scott Harris encouraged her to try again and again. Eventually, she "fell in love" with the job, racking up a long list of diverse songwriting credits, letting her explore her love for multiple genres. She graduated from NYU in 2015.

"One huge critique of my band at the time," she revealed, "was we'd have a reggae song and then a pop song and then a kind of hip-hop sounding thing, and people would come to the show and be like, what's your sound?"

She explored her love of reggae, for example, by writing with Sean Paul. Working with artists as varied as Shawn Mendes, Camila Cabello, Alessia Cara, 5 Seconds of Summer, and Noah Cyrus helped round out her musical palette.

Just as Harris noticed her talent, so did producers; in 2013, she signed with Dr. Luke's Prescription Songs company, which she still remains under. (As for his ongoing legal dispute involving Kesha and her sexual assault allegations against him, Warren said, "I wasn't around when any of that happened so I — I don’t know, I can only base on what I know about Luke, and he's a great guy and he takes care of all of us.")

Two members of The Betters went on to join MisterWives, and Warren has zero regrets about their time together. In fact, "Until You Were Gone" — the first song she co-wrote and sang for The Chainsmokers in 2015 — was inspired by her former band.

"The song was just about, do I regret this? Have I made the right decision?" Warren said. Little did she know her next Chainsmokers collaboration would take her all the way to the Grammys.

Emily Warren & The Chainsmokers

If the 2017 Grammy Awards were like Mean Girls, perhaps the statue for Best Dance Recording would be broken into five pieces, with each one given to the five artists who helped create the winner, The Chainsmokers' "Don't Let Me Down." Sung by Daya, the tune was actually penned by Taggart, Warren, and Harris at Taggart's New York apartment. The Chainsmokers are now famous enough to work with whoever they want, Warren pointed out, but they keep coming back to her.

"We feel comfortable around her," Pall explained. "She brings out the best in our talents and makes the whole writing process really fun and natural. It's hard to write good music, and we don't have a lot of time anymore, so when we do have studio time we want it to be with someone who we know already understands us and our vision, and we also know what she is about."

In other words, Warren doesn't let The Chainsmokers down. And with every catchy, chart-topping song, the duo gained more and more attention — and scrutiny. Even with Warren's background vocals, "Paris" firmly positioned Taggart as the lead singer and confirmed that his step into the spotlight on "Closer" wasn't a one-time thing. Now, he's doubling down and taking vocal lessons while Pall learns piano, proving they're serious about taking their second album to the next level.

"I know a lot of people criticize Drew's voice, but I actually think he has a great voice," Warren said. "Because artists like Britney [Spears] — not that he has Britney's voice — but you hear her voice and you know it's Britney. And I think you hear Drew's voice and you know it's Drew. That, to me, is what a good voice is — not, like, runs or anything."

Last year, The Chainsmokers' reputation as the "frat bros" of summer blew up thanks to some controversial quotes in two notable magazine stories. The hate doesn't make sense to Warren, who lived on a bus with Taggart during the Memories tour. She describes both him and Pall as "inclusive and generous," citing how they recruited their touring band via YouTube. More recently, they've crashed a high school prom and invited fans onstage.

"There's a lot to be learned from [Taggart] not letting anything beat him down and following his gut," Warren said. She kept this lesson in mind when she took the next big step in her music career.

Just Emily

It was only a matter of time before Warren decided to sing and record music under her own name. Her first two solo releases, "Hurt By You" and "Something To Hold on To," sound nothing like the songs she's co-written for The Chainsmokers. They're not supposed to. She wanted to create something more stripped back.

"It's an expression of me," Warren explained about how "Hurt By You," released in May 2017, came together. She filmed the music video at Sun Studios in Memphis, Tennessee, where Elvis Presley and Johnny Cash recorded. As for the deeply personal lyrics, she was inspired by her current boyfriend, as well as her parents' and friends' past relationships.

The "Hurt By You" chorus — "I hope you don't hurt me, but if you do, it'd be worth getting hurt by you" — mirrors her real-life struggle to let her guard down. "It's still worth making yourself vulnerable to fully fall in love with someone even though studies show it's not going to work out," she said. "But hopefully it does."

No matter who Warren's writing for, herself included, it all comes down to honesty. That's what she learned in the tear-filled songwriting session that yielded "Capsize," her 2016 collab with Frenship. To date, the song's garnered over 350 million streams on Spotify alone, a viral success Warren partly attributes to its authenticity.

"What I learned from that experience was we all wrote about something really personal and true and it struck a chord with a lot of people," Warren said, adding, "It made me have a lot of confidence in, like, if I make something that I think is cool and I'm not worried about a label thinking it's cool or fans thinking it's cool or whatever it is, [as long as] I like it, then if people don’t like it, it's OK."

So far, she's had great taste.