Documentary Conventions.

Voice over
A voice over will usually be someone talking over a video, this is usually done with narration. Having a voice over helps with giving viewers more knowledge on what they are watching for example in nature programs someone will usually be narrating on the animals and their behaviour and what is happening. Or it can be just to give their own opinion if its an influential show.

Realism
Actuality footage is used to create a sense of realism. Real is footage of events that have been recorded by cameras. This is most common with things like cctv cameras, mobile phone devices and similar things like that. Basically the things that are shown to viewers are things that haven't been changed, edited or set up, good examples of these types of videos are things like disaster events like earthquakes and robberies.

Archive  Footage/Stills
This used to help show or broaden the video more. If the video is referring to something or footage of another sort is needed then things like footage and screen shots will be taken from somewhere else if the film maker cannot get the footage them selfs or in rarer cases things that are not filmed by camera. Another reason of doing this is that it keeps the film creator from having to film something if they can just get the same footage from another place.

Interviews With People And Experts 
Interviews with people and experts are used in different ways. The most common place to see a interview would be on the news, this is done because it helps get further information on the subject/ topic. News stations and interviewees will be interviewing people like eye witnesses to get a personal opinion or word on what actually happened. Experts can also be interviewed to get a professional insight and bring authenticity. The other common way an interview would be seen would be if someone was promoting something like a new film or book. When someone is promoting something it benefit both party's because the interviewee gets their products advertised but also the interviewer also gets more views because people want come to see the different interviewees.

Expository

This type of documentary is where they speak directly to the viewer, often in the form of a authoritative commentary employing voice over or titles. Proposing a strong argument of point of view. Images are often not paramount, they exist to advance the argument. These types of documentary's should be rhetorical, and try to persuade viewers often goes by the voice of god type of thing. Images are insistently pressed upon viewers to read in a certain fashion.

Observational
Observational documentaries and programmes is where there is little to none interaction to things on film. There is often hidden cameras so the subjects are not aware of the fact that they are being filmed. Nature shows are common at being observational all as they are not really wanting to interact with animals or disturb them but to simply film them as they regular be. There are also documentary shows that use this style like "Lift". Lift is a short film that has a guy in a elevator filming people he does not say anything or interact with people in any way, he is simply there to film them and see how they react. The point of these thing is to get a different prospective on things and see things in their natural form which cannot be done sometimes if people know they are being filmed.

Interactive
Interactive documentary's lets the viewers take part and join in. These styles usually have more than one media form. This could be anything from interactive screens, buttons, pictures, animations and text.

Reflexive
Reflexive documentary's are similar to an interview as the film maker or narrator will acknowledge the people they are filming. This is done because it allows the creator to make the audience more aware of things like problems in production. These types are documentary's are not as common as other one and tend to be more experimented on, trying different things. They also often tell the truth the most as they concentrate more on the realism of things and often show footage in the way it happened. When the film maker addresses viewers it is common that viewers opinions will be swayed to the makers intent, this can be done by showing them something and talking about it in a certain way.