Do it when it's appropriate and do it in a way that does the least offending. Ie, in private, and not in front of anyone else. During my residency training there was some annoying cunt who talked non-stop, and she'd talk about other residents and it made me uncomfortable because I don't talk about people behind their backs, or talk about people period, so I didn't want to be associated with that shit. I just said "It makes me feel uncomfortable when you talk about people, please don't do that when I'm around." I try to frame shit as how I feel, instead of insulting them (ie, instead of "please quit talking about people, it makes you look like a retard.") Can't argue with how I feel!