LEV582 At times, one who listens to lashon hara or rechilus transgresses the positive commandment of הֹוכֵ֤חַ תֹּוכִ֙יחַ֙, “You shall surely rebuke” (Vayikra 19:17). For example, if one sees that someone is starting to speak disparagingly about another person, and he knows that his words will have an influence on the speaker--or even if he thinks that there is a chance that his rebuke may be effective—then halachah requires him to rebuke the speaker so that he will not carry on with his sin. Accordingly, if he allows the speaker to finish his derogatory account about the other person, he certainly violates his positive commandment. (Even if one resolves while listening, to rebuke the speaker after he finishes his story, he is still violating halacha. Would a person watch his fellow Jew eat pork, Heaven forbid, and allow him to finish eating, while resolving to rebuke him afterward? According to halachah, one is certainly obligated to reprove the person immediately, for perhaps he will listen to him and stop eating. By doing so he can spare the person from several transgressions, for each and every k’zayis he eats is considered a separate transgression. The same is true with regard to speaking lashon hara, because each derogatory statement that a person makes is considered a separate transgression. An exception can be made, however, in the following cases. If one realizes that if he allows the speaker to finish his story, he will then be able to use the story itself to show the other listeners that the speaker was merely propagating false rumors; or if halachah a requires one to allow the speaker to finish because he is relating information that may be relevant to the listener; then the listener may allow the speaker to finish … but if one realizes that the speaker is just mocking or ridiculing the other person, then it is definitely a great mitzvah for the listener to swiftly rebuke the speaker and stop him from continuing his disparaging remarks.)