Say "NO" to Vertical Video Syndrome
■ A-roll is referred to as “the talking head” because it is the footage of the main interview.
This is where you see the person responding to the questions being asked by the interviewer.
■ B-roll is the support footage that is used behind the sound of the A-roll. B-roll helps an interview move along more efficiently. It also provides context to what the person being interviewed is saying.
Most importantly, it makes the documentary less boring to look at. No one likes to look at a talking head for five-ten minutes straight.
In fiction films, it establishes setting and important objects for the audience to see. It's the camera's way of saying "look here" and "look even harder!".