Obviously every interview needs sound. But the kind of sound that people might not think about in an interview is natural sound. If you are outdoors this would be the wind, or the running water in a stream. you might not realize it at the time but sound is one of if not the most important item in an interview that makes you feel like your there. When you are filming you must figure out a way to incorporate this sound into your interview, because without it the audience would be lost.