The singer with a club band I was in used to ask first, were there any birthdays? Second, were there any requests? Third, would anybody like to get up and sing a song? We kept trying to stop him, but he just kept on doing it. So either we were lucky and they asked for something we knew (10%) or we couldn't play whatever it was (90%). And of course the occasional singer from the audience was almost always awful, and would almost always be singing to a song that we would be busking.