Lyrics and melody
You may have no lyrics at all (in this case it’s an instrumental song in fact), you may have only few words, or you may even have lots of them. When writing your hit and slow songs, you had to be quite careful with your lyrics, as they had to fit a certain pattern.
This is no longer the case when writing fillers, which means that you can come up with all kinds of crazy ideas. It also means that you don’t have to try hard to find a super catchy line for your chorus, and none of your lines must act as a hook.
You don’t have to write a melody that’s easy to keep in mind, and your melody itself also doesn’t have to act as a hook. You may even come up with a melody that’s not repetitive at all.