Conventions In a Documentary

Voice overs within a documentary.
the voice over will normally be authorized in some way encouraging the audience to think that they either have some kind of specialist knowledge or, as in the case of the people like Michael Moore and nick Broomfield: 'the right' opinions that people should pay attention to.
Example: Micheal Moore Bowling for columbine.

Real footage of events,
Documentary is essentially seen as non-fiction although there are debates around surrounding this.
however a convention of documentary is that all events presented to us are to be seen as real by the audience
documentaries often go to great lengths to convince us that the footage is real and unaltered in anyway, although editing and voice over can affect the reality we as viewers see.

Technicality of realism
Including natural sound effect and lighting to emphasise emotion and drmatisfy what is happening Nick Broomfeilds use of this in 'biggie and Tupac' when they 'run out' of sound

Archive footage/ stills
To aid authenticity and to add further information which the filmmaker may be unable to obtain themselves.
Example:

1962

USA

Black Fox: The Rise and Fall of Adolf Hitler

Louis Clyde Stoumen

I have chosen to use this as an example, as within many WW2 footage, there is only a linited amount. This means many different documentaries that are based on the same subject matter with all use simular or the same footage.  
Interviews with experts
used the authenticate the views expressed in the documentary. sometimes they will disagree with the message of the documentary although the filmmaker will disapprove them in some way.

Sound
listen out for the use of non-dietetic sound. why has there been music added what effects does this have? is sound used as a bridge between scenes and if so what meanings are made ?
example would be: "surprise me" - how much does the use of childish music undermine McDonald.

Set - Up
not just reconstructions of events that have happened in the past but also setting us typical scenes.
so if you want to quickly convey classroom you might ask a class to put their hands up likes there's a lesson in actions and the teacher just asked a question. strictly speaking what you are showing inst true the teacher didn't ask a question, it is a cheap way of getting footage a crew might have to wait 15 minutes if they waited and wanted the scene to be natural. there is an issue however because crews make a habit of using set ups thy will only using images of reality that audiences already recognize. producing fresh images and ideas is impossible. theirs a sort of viscous cycle here. if you show radically different images from inside a school you may reject them as atypical or unreal but f i can only offer you a reality you already know about how i can change your opinions.

Visual coding.
Things like mise en scene and props. is that the doctor any less a doctor if shes not in a white coat or wearing a stethoscope, or a builder in a high visibility jacket and a hard hat with workmen boots. has someone been ambushed in the street to make then look shifty.