Almost any subject has a history or a background and this should be researched extensively while writing. The most important thing is fact-checking, because a documentary with false information is doomed from the start. Use multiple resources for facts; remember that not all information on the Internet is reliable. Old newspapers, libraries, and town historians can provide all types of information as well as experts in the field your documentary is about.
A big part of documentaries is the interviews. Documentary script writers should be able to create compelling questions that elicit the best and most vocal answers possible. You should create a list of questions that allows the interviewer to ask more than enough questions so there is extra footage. You should never ask a question that can be responded with a simple "yes" or "no" response. For example, "Is riding a dirt bike exciting?" can be changed to "How is riding a dirt bike exciting and what emotions do you go through while riding?"
A lot of documentaries feature voiceovers. The voiceovers should be written in an intriguing style that pulls the viewer in. The opening line should be a line that captures the audience's attention. Try to write the script in a natural voice rather than a narrator just listing facts. Focus on the drama of the story to make the movie really come alive.
While the interviews and footage will take up much of the production, the writer should prepare the project by writing a lot of B-roll footage into the script. B-roll footage is shots of extra things like buildings, landscape, and other locations that the voiceovers and interviews will be put to. For example, a documentary on pro wrestling should have a script of B-Roll shots like an empty wrestling arena, fans arriving, fans waiting in line, fan signs, wrestling moves, wrestling entrances, wrestling injuries, wrestlers on the road, wrestling advertisements, and people watching wrestling on TV. It is up to the screenwriter to plan these shots in order to make the production more interesting.