When you are Shooting an interview in your film, you are really declaring that you are taking an active part in the making of this film and you are not just “A fly on the wall”. There are no objective questions. When you ask a question in a documentary interview, you already have the answer in it. Remember! When you do an interview you are not only trying to get the facts, you are also trying to get a point of view and an opinion on all subject and to deliver it to the audience. You have to do a lot of planning and preparations.
You can read my post about Editing for The Documentary, but for now, the important thing to know is that when you edit the interview, usually the director will choose to cover the image of the talker with other images that are somehow related to what he is saying. This is a great technique because sometimes just to see someone talking might be boring. The images can also support what the interviewer is saying and to give you more artistic ways to release your movie from the formal talking head style. The main reason is that covering his image will allow you to edit what he is saying without the audience noticing.