I can't smile while singing with others and/or accompaniment, and therefore listening hard as well as concentrating on producing a beautiful noise. I also tend to roll my eyes which looks really odd as they're crossed and thus at one point go in opposite directions. The net result is one of the many reasons why I'm an editor and not a star of stage and screen. While playing the guitar or the mandolin, I concentrate really hard and thus am generally not aware of what my face is doing until the song or tune is finished, at which point I look startled.

When singing a capella, on the other hand, it feels as easy as talking and my face is far more obedient. To make the song "go over", I generally find someone to sing to in the audience -- preferably someone who looks interested!