It is key at the start of a documentary to capture the audience attention as quick as possible. There is many ways of doing this.
First of all you need to make the point of the documentary clear from the start in order for the audience to understand what it is going to be about. For example at the start of a documentary they will ask the question why are British teenage pregnancy on rise ? or why young adults drink lots of alcohol? It will probably show some footage of what the show is about and snippets off people that will be interviewed in the show. Also at the start of the show they might have some music they is relevant to the documentary(it must not override the voice over used).
Over codes and conventions:
Actuality- Filming real events as they are happening. This could be following soldiers fighting in the war or filming a storm stuff like that is hard though that is why archive footage is important.
Fly on the Wall- This is where you film people doing real in their lives e.g follow the average day of a solider see what happens in the day shows ups and downs.
Voice Overs- Used in most documentaries have one. It tells what it is about and gives us key info about the topic
Graphics- Usually use graphics with texts, maps, drawings, still photographs in the opening sequence.
Interviews- An expert interview with someone who has knowledge of your topic. This is vital as you need someone who can give a good side of the story instead of some average Joe of the street.
Vox Pops- This is a random interview with someone ordinary of the the street used with a mic and hand held cam.
Talking Head- A shot of someone talking to the camera as a presenter of the documentary.
Music- Critical for the opening sequence but should not dominate the voice over needs a clear balance.