Framing your subject properly makes it much easier for a viewer to follow along with the video. Having the subject to high or low in the picture can make them look drowned out which can lead to viewer drop-off. The term for the space above the subjects head is called headroom. Here are three examples of headroom.
Too Much Headroom
Too Little Headroom
It’s also good to think of your screen being split into thirds both horizontally and vertically. Typically when shooting you want to aim to have your subject centered to one of the vertical lines in the viewfinder. See Below for Example:
As you can see we have our subject to the right of the fame looking to the left of the camera. To achieve this you just need the subject to keep their body facing the camera or the interviewer and have them sit between 1-3 feet from the side of the camera.