Dear person who isn't in the choir but can't help singing along:
(1) I understand that you can't help singing. I'm made the same way.
(2) I do mind, but I don't object.
(3) But please don't fill in missing parts, because this music is DESIGNED for incomplete choirs and if it's different from what we practiced I get confused. If you must sing (and again, I understand that you must) sing a part that's already being sung, for instance the alto part because I know you're an alto.