Camila Cabello Talks 'Too Good to Be True' Duet With Shawn Mendes & '99.9% Done' Fifth Harmony Album

Joseph Llanes
Camila Cabello photographed on Dec. 4, 2015 in Los Angeles.

"It's cool that he and I wrote it together and it wasn't something that was just handed to us," Cabello says of "I Know What You Did Last Summer."

When Camila Cabello and Shawn Mendes sat down and wrote "I Know What You Did Last Summer," they were just riffing in a dressing room, imagining a story about a girlfriend cheating on her boyfriend and how to turn it into an emotional duet.

"I didn't even think that it was going to make it on his album," Cabello told Billboard's Pop Shop Podcast (Cabello's interview starts at the 30:16 mark below). "I didn't expect for it to come out of that dressing room. Because I've never done anything like that before. I've had writing sessions with people, but I've never had one where you're just there and you start making a song and then it's too good to be true that something really cool will come out of this."

But something really cool did come out of that dressing room: The duo have already performed the top 20 hit on the Billboard Hot 100 and top 10 hit on the Pop Songs chart at the People's Choice Awards and just this week on Ellen. "It's cool that he and I wrote it together and it wasn't something that was just handed to us. I think that's why it's been so fun and why I get so excited that we're getting to play it at these cool places. … We had our hearts in it. That's what makes it special."

Pop Shop Podcast: Camila Cabello Interview, Grammy Wrap-Up & Future's New No. 1

This is Cabello's first song outside Fifth Harmony, the girl group that was formed during season 2 of The X Factor and earned a passionate fanbase with debut album Reflection and its biggest Hot 100 hit, "Worth It." The group will soon release a yet-untitled sophomore album, which is "99.9 percent" done, according to Cabello.

"The fans are going to really, really love it. … It's just a level up," she said of the project, which includes songs from Meghan Trainor and superproducer Max Martin. "The songs are better. Everybody gets to sing a lot on this album, which I think is really cool. … Every girl has a moment."

For more from Cabello's interview -- including her thoughts on pal Taylor Swift's magical Grammy night -- listen to the Pop Shop Podcast above!