In his liner notes for the mixtape, A. G. Cook said that:

Finn & I are obviously harmony-led, and the first thing we did was to add & change chords throughout the song, just to really make the vocal melody feel as dramatic as possible. We found that part of the process fairly easy, quite fun, but the production of the track changed quite a few times. Our initial instinct was to make it really soft/intimate, almost-but-not-quite acoustic. Charli always saw it as a much more epic, large-scale pop song, but it took us a while to really bring it there, and our first few versions were all still pretty twee! Then, very close to the mixtape deadline, we had a super late night studio session where we just added tons of stuff and made everything more aggressive. We even added this sort of thumping pop piano to the chorus, which we realised made the arrangement sound quite similar to Roar! Charli’s melody is really different, but you CAN actually sing Roar over the chorus, try it…