Songfacts®:

• Mendes started writing this love duet with Fifth Harmony's Camila Cabello when the pair met up backstage at one of Taylor Swift's 1989 World Tour shows. In the summer of 2014, Mendes and Cabello were opening acts on Austin Mahone's tour, but they didn't spend much time together. The next summer, they got together at the Swift show - Mendes was an opening act and Cabello went to see it with Ally Brooke of Fifth Harmony.
  
  Mendes pulled out his guitar and they started playing around, which led to this song. "Working with Camila was amazing," Shawn told MTV News. "We wrote this song backstage so spontaneously."
• In a behind-the-scenes look on the making of the song, Cabello admitted that the song was an emotional one. "It's basically a conversation between two people in a relationship where it's dying but nobody wants to admit that it's dying," she explained.
  
  Elaborating on the specific inspiration in Rolling Stone, Cabello said that she had been talking to a guy she liked, but found out from one of her girlfriends that the same guy was flirting with her as well. "It's about finding out that somebody had been cheating on you, and the song is that moment right before you confront that person where you both know what's next, but it's killing the both of you," she said.
• I Know What You Did Last Summer is the name of a classic horror movie from 1997 starring Jennifer Love Hewitt. Cabello and Mendes claim the song has nothing to do with the film, and the title is pure coincidence. They didn't realize it was a movie title until after they started recording the song: Cabello was born in 1997 and Mendes was born in 1998.
  
  In the film, a group of teenagers accidentally kill a guy; in the song, the transgression is cheating.
• Shawn and Camila performed the song live for the first time during the November 20, 2015 episode of Live with Kelly and Michael. Afterwards Camila expanded on how they randomly wrote the tune backstage at a Taylor Swift concert. "We were just kind of jamming out. We weren't really consciously writing a song," she said. "We kind of started spitting out melodies and put lyrics to it; I don't think we knew we had a song."
• Camila Cabello became the first member of Fifth Harmony to chart a solo hit on any Billboard chart when this entered the Hot 100 at #97.
• The "I know, I know, I know..." refrain is similar to what Bill Withers sang on his 1971 hit "Ain't No Sunshine" - so similar that Withers was given a songwriting credit on this track.
• This song was produced by Ido Zmishlany and Noel Zancanella, who also got songwriter's credits. Zmishlany worked with Mendes on several of his earlier tracks, including "Life of the Party" and "Something Big." Zancanella's credits include "Neon Lights" by Demi Lovato and "Ghost by Ella Henderson.
• Camila Cabello was surprised when the song ended up on Handwritten Revisited. "I didn't expect for it to come out of that dressing room," she told Billboard magazine. "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."
  
  "It's cool that he and I wrote it together and it wasn't something that was just handed to us," Cabello continued. "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."
• Mendes and Cabello started dating in 2019. Working on this song is when their first sparks of romance appeared. Cabello says it's when she first bonded with him as "more than a friend."