“I’M used to being interrupted,” President Obama said Tuesday night in his second debate with Mitt Romney, an event in which each man repeatedly cut in while the other was speaking.

The debates this year might be most remembered for the frequency (and ferocity) with which the candidates have interrupted each other. Nearly all commenters on the phenomenon seem to assume that it is self-evident when an interruption has occurred and who’s at fault, and that interrupting violates the rules of conversation. But just as conversational styles vary widely by gender, ethnicity, geography, class and age, so do ideas about what constitutes interruptions, and whether and when they are good or bad.

The moderators know this. Critics lambasted Jim Lehrer for not interrupting the candidates more, seeing his not doing so as evidence that he’d lost control. But if his job was to get them interacting with each other as in a conversation, then the more they interrupted each other, the more successful he was.

Yet the criticism no doubt emboldened Martha Raddatz and Candy Crowley to interrupt the debaters for going on too long or off topic. It’s a delicate business, because a moderator who interrupts risks being seen by viewers as rude. When Ms. Crowley told Mr. Romney she would “get run out of town” if she didn’t stop him, she not only stopped him, but cleverly put the responsibility for doing so on others. The fact that these moderators were women complicated the challenge for the debaters, who were mindful of the need to appeal to, and not offend, female viewers.