- For your subjects to take you seriously: If you have no idea what your talking about your subjects may give less informative and fruitful answers in an interview.
- To be able to ask good questions in an interview: Without researching a topic (and subject) you won't know what questions to ask, or ask good questions. When you know more about a subject you can ask more in depth and quality questions as you have the pre existing knowledge.
- To add more flesh to your story: Without knowing much about topic, your documentary will be empty apart from interviews. After researching a topic it makes it easier to put your on perspective on a topic, as well as being able to back up your opinions and perspectives.
So it becomes obvious that research is key to creating an entertaining and persuasive documentary.