Johnathan Paul of Premiumbeat posted a nice blog post in which he summarizes a few tips for documentary film productions. He reminds that documentary film presents challenges that are not present in the case of narrative film, because very often you need to craft your story with interviews, archival footage, sometimes you need to shoot in unsecure locations and so on. The tips listed by Johnathan should be useful if you plan to make a documentary, so make sure to check them out. For example when doing an interview remember to utilize the environment, because in some cases you might get better results when you don't focus just on the subject - especially when you are shooting a documentary on a specific event. Also remember that variety is important - you should consider how you will connect your takes in edit, you want your film to be interesting, so think how you will frame your shots, what will be the camera movement, etc.
Other things you should take into consideration are lighting (interesting video on this topic above), also don't forget that sound can be an important element of your story, so try to record the natural tone of the environment - even if you don’t think you’ll ever use it. Additionally Johnathan mentions the Rule of Thirds and he also advises to upscale the archival footage if it doesn't match the original shots you're capturing.
For more head over to the full blog post (here).