you're always allowed to be for real, regardless of whether the other person does or not. but i wouldn't suggest offending the vicar (which always makes me think of miss marple stories :P) if it would in turn piss off your parents. plus, if he's just lost his wife, he's probably in a bad state already. you wouldn't want to push him over the edge or anything.
and sometimes being nice to other people, even if you don't want to, can make you feel better, too. when i get really depressed, it's usually because i'm stuck inside myself, which translates to selfishness in its logical end. and now ends this installment of "ask pseudo dr. sadie."