I think a line exists between being OK talking about someone, describing what happened or how you felt about those happenings and saying inappropriate things.
It's hard to listen to people say bad things about others who they don't like, without adding to the conversation. I know often that it's not my place to correct them, but I also don't want to add to the conversation. I find myself saying, "Yeah," or "Uh-huh," trying not to add anything beyond what they say.
Often times when we talk about people behind their backs, we have to bite back our words anyway.
I am not huge talker, unless it's with someone I am very close to. I like writing, because it gives me the opportunity to think through my words and even change them after they come out. With speaking, I often don't say what I want to say or want to change the wording after it comes out.
They always say think before you speak. Sometimes though, I think silence is just better.