The four basic approaches to documentary cinema are 1. factual films; 2. instructional films; 3. persuasive films; and 4. propaganda films. Each has its own unique purpose, style and tone.
1. Factual films are intended to portray the subject, whether it be an event, person, place or thing, in a very straightforward and matter-of-fact way. It's not meant to persuade or make an argument toward a thesis. Its purpose is solely the dissemination of fact-based information and transference of knowledge.
2. Instructional films take factual a step further by educating audiences on how to do, view, or approach some particular subject. It presupposes some sort of action on the part of the viewer. It is usually fact-based but also driven by the goal of achieving some sort of outcome or result on the part of the viewer.
3. Persuasive films inherently take a side in a particular issue and present it in a way that is intended to convince the viewer of the validity of that view. It lays out a thesis and then presents evidence, whether based in fact or not, to coax, cajole, enlist, exhort, or otherwise impress that viewpoint by its conclusion.
4. Propaganda films are those that intentionally disseminate deceptive or distorted information in order to achieve a desired response from the viewer. These films deliberately eschew facts and evidence, often relying on emotion as the primary appeal, since the response is more important than the content itself.