In a manner of speaking, manners in many workplaces have been tossed out with old typewriters and carbon paper. It is not as if there is anything inherently wrong with interruptions, as there can be critical situations that call for them.
While it is easy to conclude that people who barge in are inconsiderate or self-centered, interruptions are becoming increasingly common and even acceptable in certain aspects of communication. For example, you are on the phone and a call-waiting beeps in. "Oh, let me take that call." With a click, the other person is left hanging while the interruption takes priority. Or, you are working online and receive Instant Message. It's really an instant interruption, and you can be perceived as rude if you ignore it. If these interruptions are acceptable, then what about barging into a conversation?
If this happens again, you can use a friendly tone and say something like, "I'm sorry, we're not quite finished." This is not an attack, but merely a statement of fact. Unless the interrupter has a very important message, he or she will get your message. And so will the person with whom you were speaking in the first place.
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