Idk w h a t exactly the build up here is, but (Oc/character) and (crush/best friend) are just hanging out, maybe cleaning while one of them just so happens to have a broom, and one of them starts to hum a tune. The other recognizes the song and hums along, and somehow this duet turns into a mini concert between you two and (if you were cleaning) possibly jamming with the broom I mentioned earlier like its a microphone or guitar. Basic ik