Carefully plan out your questions. Only ask relevant well worded questions that will help support the intended purpose of the documentary.
Avoid one word answers like Yes or No.
Provide all the questions to the person being interviewed in advance. This promotes the possibility of getting strong answers and better footage.
Scout out the location, if possible, to determine possible camera and character positions.
Ensure your remote audio microphones have sufficient power and are on corresponding frequencies
Test your recording equipment before you do the interview.
Know how to change the frequency/channel in case there is interference.
Create an environment that eliminate noise during the interview.
Plan several mappings prior to going to the interview as you may have very little set up time and only one opportunity to ask the questions and record the responses.
Consider carefully how you will frame the person being interviewed in the shot and how / if possible you will show the person asking the questions.
Once you start recording, do not stop and do not move the camera or change the focal length.
If you have a second camera, consider recording other imagery: hands, relevant room elements, etc.
Do you need to to record the person doing the interview?
Consider other environmental influences to the shot: light sources and angles, background elements
View all the footage and carefully trim the clips to accurately display the information provided in such as way so that the answers support your documentary but do not take the responses out of context.
Carefully consider the duration of your shots and utilize voice-overs to provide visual variety for your audience.