Be respectful of the traditions of those hosting the event, even if they differ from your own or fell unfamiliar.
Avoid inappropriate whispering, giggling, or moving about. This is not only terribly rude, but disruptive to the ritual, and you may just miss something important.
If, for any reason, you do not wish to partake of the ritual Chalice (such as allergies illness, alcohol issues, whatever) simply raise the cup in salute, thank the Deity for their gift, and pass it on. Likewise with the plate.
Be responsible for yourself. Clean up any messes you make. If you break an item, replace it or make compensation.