IMHO, then, a demo should:
*Be a good dancer. Ideally should be able to lead and follow well, and must be able to pick up moves quickly from either side.
*Smile on stage and turn up looking smart
or at least not like they've been dragged through a hedge backwards.
*Not talk on stage unless making a quick comment to the teacher about something - I've seen demos try to organise the rotation, explain moves, start/stop the music and so on without being asked, which is not what they're there for.
*
Probably be a taxi dancer, since they will hopefully be a good dancer already, and people will recognise them as such.
*
Be happy demoing. Nothing worse than being put up on stage in front of people and not being comfortable with what you're doing.
FWIW, Caz, if I was teaching I'd have no problems at all with you demoing for me

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