It could be because it affects what you're working on now or because it affects another high value activity that must be handled urgently. Don't leave it open-ended. Get agreement to the time needed and explain that this way you'll be able to get back to your deadline. Stick to the timeframe. Thank the person for his or her cooperation.
Most interruptions are of the untimely type.
Unfortunately, we treat them too often as appropriate. In most cases, probably not. Don't give up on controlling interruptions. Even if you haven't been able to control them before, you will with the methods I've outlined here and the payoff will be that you'll be able to concentrate your efforts and get your work done quicker with better results. Jim DeSena helps growing companies continue to grow.