If you are making a documentary, narration may become a part of the final product as well. It may be sparse, simply setting the scene and introducing the topic, or it may reappear throughout the film. Good narration allows the viewer to make decisions for him or herself, but it focuses attention on what is important in the film. Bad narration gets between the viewer and what is on the screen and tells them what they should feel. Well-written narration can help to bridge the gap between scenes, adding to the speed of the transitions.
Many contemporary documentarists prefer to have the voice of actual participants narrate the film, but it is not unusual for the filmmaker to deliver the commentary, perhaps in a first-person account of creating the film itself. It is even possible to use narration ironically, so that the difference between what the viewers hear and what they see deliberately undermines the credibility of the commentary.