What need to be prepared for the Interview?

Conduct the subject to have a pre-interview first

A pre-interview is to have a long time conversation with the person over phone or face to face before start filming
This can let the filmmaker more familiarise with the subject, let the subject get used to the environment first, but also to let the subject feel less stress and more comfortable with the interviewer and front of the camera.

Before the documentary interviews begins, filmmaker should've know what the interview footage is going to look like, what kind of background will be used? Where will the interview will be? How's the lighting will be set up? Three point lighting? Natural light?
Being prepared and aware of the interview setup before filming will help the filmmaker avoid any accidents and will save more time for the filmmaker, which will also let the subject feels professional and confident for the interview.

Traditionally, documentary subjects in a sit-down interview will be looking at the interviewer, not the camera, so that they are slightly looking to the side of the lens.
Sometime there will be two different camera angles, a close up and a medium or wide shot. Therefore during editing, there will be more options and create more contrast.

4 ways of processing the interview

Interview often start with a fairly general, open-ended questions, to let subjects answer questions more thoughtfully. These questions might not even have to be about the interview topic, could be about their childhood or life experience. Because these broad interview questions can often lead to more complex when begin to talking about the interview topic

As the conversation develops, filmmaker have to stay flexible, it's okay to go off the script.
While listening the subject, always have to attuned to their emotions and interests, pursue lines of questioning that occur to you in the moment.
When subject doesn't feel comfortable with the topic or question, or the subject is unable or doesn't want to answer a question, move on to the next question, if possible, circle back later.
The goal is not to just get a list of answers, the goal is to create the right conditions to get a fantastic answer and keep your subject engaged.

Don't ask question that can be answered with a yes or no response, because you want your interview to be in depth and the subject to be able to speak at a length on a topic.
Instead of asking 'do you use social media?', ask 'what was your experience of social media?'.

If all the set-upped question doesn't take the interview to anywhere, let go of it. Throw out any preconceptions of how you though the conversation should've go, and take a step back.
Listen for what the subject really wants to talk about, and then improvise try to circle back to the topic.

Reference: https://www.masterclass.com/articles/how-to-conduct-a-documentary-interview-top-interview-techniques-and-tips-for-documentary-films-and-tv