If your documentary film is built on direct cinema footage you may want to maintain that style in your interviews.
Direct cinema, sometimes in the form of the observational cinema verite, is a way to make the audience feel as though they are observing events as they are actually happening. Instead of using music and interviews to try and give the gravity of a situation, the viewer is invited in to watch and feel as they would naturally. Of course this is a filtered perspective as all filmmaking is, but it is still meant to give the appearance of uninterrupted involvement. If your film is focused around direct cinema, which watches as events take place, then you may not want to interrupt that feel to have interviews that are framed and executed in classical film fashion. The majority of the footage is more “on the spot," and therefore cutting away to studio interviews can break from the rhythm that you have established. In this situation you can make your interviews fit the aesthetics of direct cinema while still being as prepared as any.
The best way to do this is to minimize the appearance of set-up as much as possible. Instead of conducting the interviews in any type of studio make sure that they are done in an environment that has been used previously. This can work as someone’s home, an outdoor location such as a porch, or while walking. The point of this is that you do not want to remove them from the spatial reality that has been established in the primary direct cinema moments. You can still use things like a light kit, but the light needs to be so diffused that not even the most attentive viewer will be able to pick up on the set-up. You want their faces to be lit enough to look good on film, but not enough to make it look dramatically artificial. Instead of bringing them far away from their background you can push them a little bit closer, which makes it look even more spontaneous.
A Bird in the Hand
Since the majority of the direct cinema footage will likely be handheld or non-stationary in some way you may want to continue this for the interviews. Try having the director of photography use a handheld camera, but place them in a still spot such as against a wall. This will let the image remain relatively stable but still have a little bit of shake at the edge of the frame. That will allow it to continue to look as though it is happening in a comparative fashion to the direct cinema, but the image will still be good enough for the audience to maintain a consistent visual of the person in question.
Though it may seem natural to involve the interviewer in the video if it is done in direct cinema fashion this will disorient the audience even more. Direct cinema is meant to give the impression of an uninvolved onlooker, so the interviewer must never get in frame. If you are doing the interview while moving this is even more difficult, so make sure that the interviewer is placed far behind the camera and that the subject is well prepared on how to answer the questions using full sentence answers.
You do not need to forgo quality audio at all, in fact that is not advisable. Keep using a lavaliere microphone, but make sure that it is well hidden. If you are moving you should go ahead and use a wireless microphone so that they can maintain free motion.