You are sitting at someone's table in a party or at a reception when people start speaking negatively about somebody else. In Judaism talking bad about other people is considered a very serious sin known as lashon hara'(the wicked's tongue). What do you do in that situation? You tell yourself: I won't say a word. But listening to leshon hara' is as bad as speaking leshon hara', because by listening you are seemingly approving it. Moreover, listening might be considered even worse that speaking, since you are giving an audience to the speaker.
How can you avoid this transgression in such circumstances?
You have a few options:
1. You can rebuke the gossipers--making sure to do it in a respectful way. You can remind them that this is a serious Tora prohibition and that they should change the subject.
2. If you know that they will not listen to your words then, you should consider to get up and politely leave the table.
3. If you find this impossible, then you should prepare yourself to stand firm so that you will not be guilty of a great sin. Make sure to fulfill the following requirements: a. Try to not paying attention to the conversation and if you do, decide firmly in your mind that you will refuse to give credibility to anything bad you hear about any person. b. Make sure that your facial expression does not convey any hint of approval to whatever is being said. If possible, your expression should convey strong discomfort with the whole conversation.
The above applies if one is innocently sitting at one's place when the leshon hara' conversation is taking place. However, if someone strolls through an area where he overhears such a conversation and stops to listen, that is considered a willful sin, even if one takes no part in the conversation and does not approve of it.
Adapted from: "Chafetz Chaim, a daily companion"