When making a documentary there are a number of ways you can approach any single subject, and each approach will give your documentary a different feel. Taking a look at documentary genres and sub-genres, such as observational and participatory, can help you in determining the best way to approach your topic.
Observational documentaries follow a fly-on-the-wall approach. This technique focuses on observing the subjects in typical, everyday situations. As an observational documentarian, you're strictly there to capture what happens, without influencing the outcome in any way. Frederick Wiseman's High School (1968) is a great example of this approach.
A participatory approach is the exact opposite of an observational one. If you choose a participatory approach you are choosing to step out from behind the camera and actively participate in the documentary you're making. This approach can be found in documentaries like Bowling for Columbine and Super Size Me. The topic is explored through the director's experience and point-of-view, and focuses on persuading the viewer.
As a documentarian, you determine the viewer's experience. Whether you choose observational, participatory, or one of the many other techniques, your approach will significantly affect how your viewer perceives your documentary and the information presented to them. Finding an approach that best suits your storytelling goal and style can increase the overall impact of your documentary.