In a nutshell, if you do not know the song, or if you are very new at playing for someone that has a celebrity standing, don't get on the instrument. Be humble. If you can't do it, you can't do it. There is no shame or embarrassment in that. It should motivate you to practice and learn as much as you can about music ministry.
As for dissing musicians...there really is no room for that. Just be gracious, but serious. Be Blessed!!!