I got paid to do that once. Seriously, I was paid to follow somebody around and either improv or cut into songs I knew by heart that correlated with whatever they were doing.
They entirely did it because they wanted to know what it would be like to have theme music all the time.
Luckilly they were into acting and just stayed home most of the day (Except when they decided to go to the park for a minute, but good for me nobody was there...), seeing what would happen if they looked angry... or sad.. . or apathetic...
I was really scared that they would go to a library O.o. But I was safe. :P.