The two lynda courses linked below are essential for anyone shooting documentaries or interviews. It includes invaluable information about how to set up lights and position lights to present your subjects in a flattering manner.
3-Point Lighting: A classic lighting setup for film and documentary.
The "key light" is the dominant light source in the 3-point lighting system. Position the key light first, either 45-degrees to the right or left of the camera and slightly angled down onto the subject.
Next, set up the "fill light" on the opposite side of the key light. The fill light's purpose is to soften some or all of the dramatic shadows produced by the key light.
Lastly, set up the "back light" behind the subject and off to one side. The back light helps to separate the subject from his or her background and can be used to accent the subject's hair.