(recommended: Words of persuasion)
(optional: other musical instruments)
The volume of the audience during the chorus (or repeated sections).
none I can think of
Time to play:
2-12 minutes (basically, slightly more than the length of the song)
tip1 - if the audience sings a section that you DO NOT (maybe echoing a portion or maybe a chorus that you shut up for)
tip2 - it can help to quickly reinforce the idea of crowd participation before your song. Some people aren't sure if it's OK.
Pleasure of pleasing others.
Different audiences make for different play experiences.
Audience can give surprising responses, necessiating quick thinking.
How well this goes can depend heavily upon audience. A room full of musicians is nearly always more eager to join in.