How should one who hears lashon hara respond?
I've read a lot about lashon hara from the perspective of (not) speaking it. My question is from the other side: what are one's obligations, both halacha and ethically (per Judaism), upon witnessing it? I understand that we're supposed to not believe it, but are we supposed to take other actions?
For example, in a conversation where somebody else says what sounds like lashon hara, should the listener question/challenge the statement (trying to judge favorably)? Make a counter-statement (if the listener knows the claim to be false)? Change the subject to try to prevent anything more being said on the subject (and thus more lashon hara possibly being said)?
Does the answer depend on the nature of the lashon hara, like how damaging it is if believed, how many people are present, or the status of the people involved?