This is Acting is an album of songs Sia wrote for other artists, but for whatever reason were rejected. "Reaper" she wrote for Rihanna.
Kanye West wrote this song with Sia, although the collaboration wasn't very collaborative. Sia says she showed up at the studio to work on the song with West, but he wasn't there. West had left her a track and some notes, and Sia did her thing, adding melody and lyrics.
Sia doesn't have a strong connection to many of the pop songs she writes that were meant for other artists. This one would have been left for dead if her manager didn't push her to include it on This is Acting. She told Rolling Stone: "I'm not emotionally attached to it. I thought it was a fun song. I think it's a good, fun song, but I didn't anticipate it being on the record. But my manager really likes it so I put it on for him."
Sia and Kanye West also teamed up for Yeezy's track "Wolves" along with Vic Mensa. The trio performed that song during Saturday Night Live's 40th anniversary special on February 15, 2015.