Conventions of a Documentary

There are many conventions of a documentary such as-

Voiceovers:

voiceovers will be authoritative is some way, making people believe that they have some kind of specialist knowledge. or in some cases make people concentrate on the 'right' opinion.

Footage:

the footage in documentaries is normally 'real' footage of real life events because documentaries are seen as non-ficton however this is debatable wether they are all non-ficton or not. Footage taken could be from any time period depends on what the documentary is about. for example if the documentary is on an event way in the past such as world war 2, the footage used will most likely be from that time so it will be in black and white and the quality won't be as good. 

Still images are also commonly used in documentaries just to add a little bit more information or just to remind someone of a certain event if the narrator is talking about the event.

Interviews with 'experts':

they use a lot of interviews in documentaries really just to back up the authenticity of the message that they are trying to get across in the documentary. Sometimes they might disagree with the message but they add this in sometimes just so they can disprove them.

Sound:

for sound in documentaries they mostly use non-diegetic sound to add a bit of entertainment to it, because with sound the documentary would be quite boring and not very fun to watch. it can also represent different stereotypical sounds for different genres so the viewer knows what type of genre the documentary is.