When in the process of interviewing a customer, you will be very busy trying to ask questions of the customer to draw out their unarticulated needs, capturing all the customer's salient points, as well as observing subtle clues and gestures the customer makes when responding to a query. All in all, there is an awful lot going on and the challenge of your job is to collect as much of it as possible, which can become increasingly difficult with note taking alone.
Unfortunately, it is often impossible to capture everything and this is why, whenever possible, you should ask a customer's permission to audiotape or videotape the interview. Explain to the customer that you want to record the interview because their input is extremely valuable to you. In most cases, you will find that the customer is perfectly willing to allow you to do this and they often forget that the recorder is even on during the interview. If they prefer that you do not, of course, you should...