Documentary

Sheila Curran Bernard, author of Documentary Storytelling defines documentary as follows:
"Documentaries bring viewers into new worlds and experiences through the presentation of factual information about real people, places, and events, generally -- but not always -- portrayed through the use of actual images and artifacts. But factuality alone does not define documentary films; it's what the filmmaker does with those factual elements, weaving them into an overall narrative that strives to be as compelling as it is truthful and is often greater than the sum of its parts."
I would give the following definition: Documentary is a nonfiction motion film that attempts to document reality.

Expository documentaries

The expository mode was first used by John Grierson in 1927. This mode became the standard mode for a long time. It was used in nearly all documentaries until the 1960s. It is still used today, although not as frequently.

The key features of expository documentaries:

• expose a person or a topic
• speak directly to the viewer ( having a commentator talking over the video and explaining the story)
• try to persuade the viewer
• they are often historical or biographical
• didactic

Example: Frank Capra’s wartime 'Why We Fight'

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The documentary looks at why and how the United States entered World War II. 'Why We Fight' features many interviews throughout the film to expose the topic and try to persuade the viewer to support American involvement in the war. A commentator speaks directly to the viewer  and explains the story. Therefore, the documentary appears to be didactic.

Observational documentaries

The first observational docs date back to the 1960’s.

The key features of observational documentaries:

• observe  life with a minimum of intervention
• no voiceover
• no presenter
• no interviews

Example: Frederick Wiseman’s film. High School (1968)

The documentary observes the teachers and how they interact with the students in a 60s American high school in observational style - with no voice over, no presenter, no interviews.  The movie takes the viewer from one classroom, office, or assembly facility to the next. Thanks to the observational style the film is to be true. And it is the viewer who interprets the events, not the author.

Interactive documentary

The key features:

• made with the help of new technology
• use data, mapping and other content in interesting ways 

The example can be found here and here and here. One more example of interactive documentary is given below:

Man With a Movie Camera is an interactive documentary shot by people around the world. They can record images interpreting the original script of Vertov’s 'Man With A Movie Camera'. Anyone can upload footage. Every day a new version of the film is compiled from shots uploaded to the dedicated site. 
Privacy regulations can be infringed in the making of this kind of documentary. The film makers ask the public to only upload video or images which they own or have the necessary licenses, rights, consents, and permissions to use. However, it is difficult to control.

Reflexive documentrary

The key features: 

• often the filmmaker(the presenter) is a part of the film and can often be the main character
• the presenter takes the viewer through the topic and links sections of the film together while being present on screen for a lot of the time

Example:

In the documentary Ross Kemp shadows the British troops offering an insight into life of the soldiers who are fighting on the front line. He is a part of the film being present on screen for a lot of the time. Kemp is often in very real danger. 

 Performative documentary

The key features:

• stress subjective experience and emotional response to the world
•  might include  re-enactments of events designed to make us experience what it might be like to do something.
• often uses actors to dramatize events 

Example: 'Super Size Me' by Morgan Spurlock

This documentary film stars Morgan Spurlock, an American filmmaker.The film shows a month period during which Spurlock ate only McDonald's food. The film depicts this lifestyle's  effect on Spurlock's physical and psychological well-being. Spurlock directed the film as an experiment in determining the dangers of fast food. The documentary has all key features of performative films. It is based on subjective experience and emotional response to the world  to make us experience what it might be like to do something.

Dramatisation is -used to draw the audience attention to the story. It is the drama that gives us a chance to identify with the characters, what they are doing, and what is happening to them. Re-enactments bring a sense of engagement.But sometimes re-enactment can cause problems as it happened with a BBC1 wildlife documentary on grizzly bears when a faked element of a key scene was a serious breach of editorial guidelines. The scene was re-enacted, without viewers being aware of the deception, in breach of the corporation's editorial guidelines because viewers had been "misled". That is why such films are often preceded by an on-screen message stating that some scenes have been dramatised and accompanied by a programme about how the show was made.
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I think a good documentary  has to hold the audience's attention. That is why it is necessary to choose an interesting and meaningful topic. I suppose that if there is no real-life footage of an event the documentary discusses, it is acceptable to use actors to re-create the event. But it is necessary to provide the fact to the audience that the footage is a recreation. However, any documentary film should be grounded in reality.