Documentary is the film based on real life, it will show the true story of the object but its will polishing this story to make it become an ‘art’ to show the truth and nature and make people thinking about life or other things. The core of documentary is truth.
The location, situation and people in the documentary is consistent with the actual situation,therefore a documentary didn’t need too many actor, In One Year Lease there even no actor just a cat and lot of calling.
In order to ensure the authenticity of the documentary , people often quote some past images and interviewed different people as the proof , for example , in Grizzly Man they interviewed the protagonist’s friends and parents
There are different types of documentaries, but the common core is that lots of documentary films wanted to express a educational themes,most people in the documentary is the parties , so their performance is very natural , it is very easy to touch the audience.
Also, documentary can be used as a form of journalism, advocacy, or personal expression.