The historical documentary, as well as other documentary styles or original creations, often use a format of laying B-roll over interviews or other sound focused material. The idea is that someone will be talking and then you can provide footage or photos that can describe, augment, contrast, or in some other way elevate their discussion. Likewise B-roll can go well into montages, musical sequences, and even lay on top of other footage. The best way to get a great stock of B-roll is to know what it is, what you are looking for, and how to get that footage.
B-roll is different than direct cinema footage because it is not meant to be the main focal point. Direct cinema is a video of something happening where the focus is on that video. This would be an event or situation recorded and then shown on its own, where everything in that video is of the direct subject. B-roll, inversely, is meant to purely elevate that which has already been established. This can be something like a segment of a sit-down interview or even the kind of direct footage that we have just talked about. This is done for a number of reasons, but really it allows for visual interpretations in another form of whatever is being said in the interview or happening in the direct cinema video. You can give a contrasting video over it to show that what they are doing or saying is dramatic or controversial. You could simply show a visual representation of it, or even a comical rendering to break up the tension. The technical function of B-roll is to cut seamlessly between interview clips and direct cinema footage, to cut up and shorten these segments, and to make these clips seem less long. The B-roll covers up all of the transitions that are made when cutting clip into smaller sub-clips or between clips.
With this in mind you should look at what you have in the way of interviews and direct footage, especially where the transitions and cuts are. Use this as a guidebook for acquiring B-roll, finding footage that will meet the visual needs that you would like. Usually you will not be using sound for the B-roll, except at a very low level, because you do not want to overshadow the audio of the A-roll.
Maintain the energy with your camera work, moving the camera when necessary. The great thing about getting B-roll is that you often have the ability to do multiple takes, so feel free to experiment. Locations are often a huge B-roll source so get as much footage of these as you can. You may want to film a similar event to what was being discussed in the documentary, so you will have to be a little more spontaneous for this. The events do not have to be identical, but give the same emotional and contextual feel.
Keep It Focused
The most important thing to consider with B-roll is to maintain the feeling that you want the audience to get from the interviews and direct footage. Employ public domain, news, and stock footage whenever possible, but make sure that they act as a set piece in and of themselves because they will not coincide with the visual pattern that you are creating with your own footage. Creativity is always key, so try new things and treat every B-roll shoot as you would with a scene in a narrative film.