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I interview a lot of people who aren't used to being on camera. Often, they weren't even planning to be on camera that day, but rather were volun-told they were going to do a testimonial. It can be difficult to get compelling answers from someone who has never been interviewed before, but it's not impossible.
The best way to get a natural answer from an interviewee is simply to talk to them like the camera isn't even on. There are the common tricks, like not telling them the camera is rolling or just starting a conversation and never saying you actually started the interview process, but I find those aren't quite as easy or elegant as they seem on paper. But really, just talk to them and guide them through the conversation. Don't change your demeanor or suddenly stop and say OK, WE'RE GOING TO START NOW. If you do that, you'll see their posture change and their face tighten. Relax, and talk. And all those ums and ahhs will start to go away if the conversation feels natural.
Don't be afraid to coach them.
I always give interviewees a really simple rundown on how the ideal interview should go: self-contained answers, say whatever you want, if you want to start over, it's all good, etc. Even with being very concise and simple, and avoiding any camera-ish jargon, these often go right out the window when the camera is rolling and nerves hit. Don't be afraid to gently pause an interviewee and remind them you can't use their great answer because they're rocking back and forth in their chair. And if you don't want to stop them to get the answer again for some reason, you can always blame a random noise in the room for ruining their answer. Oh darn, we have to do that one again.
This is a staple of interviewing, but I've seen so much video that obviously didn't do this. Don't ask yes or no questions, or questions that can be answered with a single answer. Don't say "How long did you use Product Y?" Ask instead "Tell me about your experience with Product Y." Trying to edit a video where all the answers start with "Because.." really sucks.
Ultimately, the interviewee wants to share their passion for an experience, and they want to look good while they do it. If you present yourself as their advocate and assure them that you'll help them out and make them look and sound great on camera, they'll often speak a little more naturally and confidently. Of course, there will be the occasional person that just melts down on camera, and if that happens, just wrap it up and move along because they're probably going to self-destruct any minute.