Billie Eilish made a surprising revelation about her collaboration with Rosalia for their track "Lo Vas A Olvidar."

Earlier this week, the "Bad Guy" hitmaker took to Instagram to announce the debut of their Spanish language track for HBO's teen drama series, "Euphoria."

"You guys have been waiting for this "Lo Vas A Olvidar" with @rosalia.vt for @euphoria song and music video out thursday at 9am pt!!!" she wrote.

Now, with its debut, the five-time Grammy winner dropped some juicy details about the single.

Billie Eilish's Writing Process with Rosalia

In her recent interview with Apple Music's Zane Lowe, the "No Time to Die" songstress shared her experience co-writing the song with the "Con Altura" hitmaker.

Billie Eilish mentioned that it was a month-long process.

The Los Angeles-born singer recalled that it took almost six months for them to reach the second verse.

"We didn't get back to it until literally last year, 2020. And so that was mid-quarantine. We're just like, 'Yeah, let's work on this song.' Because in our heads, we were going to put it out in the summertime," she explained, adding: "I just was like, 'It needs something else... We wrote it was the whole song was done, except we needed this last line."

A Collab With the Whole Fam

As for the final output, the 19-year-old award-winning artist shared that it was a collaboration with her family.

Billie Eilish revealed that she sought help from her brother, Finneas, together with his girlfriend Claudia Sulewski, as well as their parents Maggie Baird and Patrick O'Connell.

According to the "Ocean Eyes" singer, this all happened over their Friday tradition--involving dinner and movie time.

"You guys want to hear this Rosalía song we've been working on?" she recalled asking her family, adding: "We all came down to the studio, and we listened, and me and Finneas were like, 'Should we just write this line right now?' So I was sitting in the room with my entire family."

As a result, they've come up with a final lyric that goes, "You say it to me like it's something I have any choice in/ If I wasn't important, then why would you waste all your poison?"

Aside from this, the majority of Billie Eilish's songs were co-written and produced by her brother--this includes her debut EP "Don't Smile at Me" and the "James Bond" soundtrack "No Time to Die."

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