Genre

A documentary is a nonfictional motion picture, usually intending to document some aspect of reality. Documentaries are often used in order  to reveal an unusual, interesting or unknown angle. Documentary filmmakers are often motivated to make their films because they feel a particular story or viewpoint is not being covered by mainstream media.
Originally referred to movies shot on film stock, documentary film has subsequently expanded to include video and digital productions that can be either direct-to-video or made for a television series.
The term "Documentary" was first applied to films of the nature of Robert Flaherty's film "Moana"(1926) published in the  the New York Sun on 8 February 1926 and written by "The Moviegoer", a pen name for documentarian John Grierson.

As I am doing a Documentary, I have researched into what conventions usually feature within a documentary.

Camera Shots:
In documentaries, at the start of a particular scene - mostly on lifestyle documentaries - there is an establishing shot to show where the scene is taking place - such as someone's house. In addition, as most documentaries include interviews, mid-shots and close ups are often used to focus upon who is being interviewed. Often in documentaries, there is no set camera shots - especially in reality/lifestyle documentaries. 

Sound:

Usually in most documentaries there is non-diegetic sound. This can be in the form of a voice-over narrator who guides the audience through the documentary, giving them background information on a specific topic. In addition, non-diegetic music is also used throughout documentaries during cut-scenes - often when an establishing shot is being shown. Documentaries also have diegetic sound which is in the form of interviews and scenes where what can be heard is what is being shown - e.g a conversation between two people or concert footage. 

Interviews:

One of the main features of a documentary is the interviews that take place. The interviews are often with people afflicted with the main subject of the documentary -e.g a documentary about a music artist will often feature journalists and other musicians talking about their work with the artist. The interviews can be in the form of a studio interview - often found in high-budget documentaries about celebrities, musicians, etc. However, in lifestyle/reality documentaries, the interviews often take place in a natural setting - e.g a documentary about a teenager may feature interviews that take place in their bedroom. 

Editing:

In documentaries, there are often straight cuts in between scenes of various lengths - usually long - as there is no need to create atmosphere as a horror film would. Another editing technique that is often used is transition screens between scenes to allow the audience to know that it is a different day/different person, especially used in reality/lifestyle documentaries. 

Archive Footage:
Often a documentary will feature both archive footage - e.g a concert or interview - and still images of the subject. 

Technicality of Realism:
The purpose of a documentary is to inform and tell the truth. Because of this, a documentary needs to include realism to make it factual.