Here is a piece that I wrote for Introduction to Film that acts as a short summary of Documentary/Film Essays:
A documentary or documentary film discusses a particular theme or topic rather than focusing on a narrative. It may include fictional or staged scenes for the benefit of the viewer as long as it does not change the events. They can be made up of different elements including interviews, archival footage as well as footage shot in the field.
One example is The Gleaners and I (Agnès Varda; 2000; French) which follows various gleaners in the quest for food and other items. The film presents a categorical argument that gleaning is a dying art and rather should be encouraged as there are many people who cannot afford or have access to food and could benefit from it. Varda does so by conducting interviews with Gleaners as well as filming her own first-hand experiences in the field.
Another example is Morgan Spurlock’s Supersize Me: A Film of Epic Proportions (Morgan Spurlock; 2004; United States). In it Spurlock categorically shows the effects of eating only McDonald’s for a 30 day period to explore the increasing spread of obesity throughout U.S. society. He, like Varda, uses interviews, expert opinions, and first-hand experience to present his point.
In both examples although the subject matter is different, the two directors approach their documentaries in a similar manner. Both open with information about their topic and introduce the issue, they present a first-hand experience of the events from their perspective providing relevant facts and then come to their own conclusion on the topic based on the facts to convince the audience of their view.