The link to light

Light is most effective when linked with some purpose, one of those being a documentary. Taking on the information learnt about the way light behaves and how it can be manipulated to serve a specific purpose I am going to understand how specifically certain lighting is used for a documentary purpose and why.
The most important and widely used light set-up is the basic three light setting, which similar to stills photography uses a light to fill in front of the camera to isolate and illuminate the subject, a second light is used to hide unwanted shadows and a third light is used to separate out the background from the subject you are shooting.
The fill light to the side completes by filling in unwanted shadows created by using a key light, after all the key light on its own would create unwanted shadows on the subject, especially for documentary purposes such as shooting a person for an interview, much like shooting a portraiture in photography.
Again when using a lighting set-up such as this rules must be obeyed for example the inverse square law which dictates where light falls off of the subject and also most of the time the lighting would be elevated to ensure the light is much more softer and gives a more desirable look.
The mood you intend to create whilst filming is also heavily influenced by how you arrange the lighting, high-key images filled with complete fill light suggest freshness and a sense of youth whereas low key lighting suggests a more sombre tone.
Of course to create certain tones you can also add textures implementing items such as soft boxes to lights to establish that softer feel, in the case of documentaries it can add warmth to a scene and get people associated with the subject or identifying with a persons view or expression. 
In summary lighting is key to a documentary whether you are using a three point lighting system or are using natural ambient light outside of the studio, to convey the message you are trying to put across when filming.