Great documentaries, whether studio or home produced, consist of two main types of footage. Interviews, often called "A-Roll," are what define most of the story and lets the audience get to know the characters. The second is called "B-Roll," and that includes all types of footage that you put over the interviews or used as “cut-aways” between them. B-Roll is what defines the quality of a film because it gives shape and dimension to the story your interviews are constructing.
Think About the Topic
The best way to plan what type of B-Roll to get for your project is based on the topic itself. Essentially you want to get the largest variety you can get. If the events you are covering are current, then you can try and capture footage as it is happening, but for most home documentary projects this is not the case. The best way to plan is to know what types of footage is available. There are photos, stock news and other types of video footage, location shooting, and many others. When selecting what types of footage you want you will need to plot out exactly what each interview clip is talking about and try to pair it with footage that will illustrate their discussion. If you are trying to do a digital video project about a historical event you may only be able to get photographs, so you should know exactly what your limits are going to be. If someone is discussing a certain event you can shoot video of the locations where it happened, show historic photos of the event, and if some related event is happening you can record it and use that to illustrate it.
Get to the Library
One resource that home producers often forget about is the ability to use newspaper headlines to discuss a historic event. Most libraries and local historical societies save newspapers and can do high-quality digital scans of headlines for a moderate price. The same goes when you are looking for photos that are not easily accessible online. Photos often have copyright restrictions on them so it is important to get clearance ahead of time, but if they were found at an archival institution they can often help you take care of those issues.
Make Sure The Audience Has Time
Once you have a lot of footage try and place it in the film so that the changes in footage you are using follows the same speed and rhythm that you have established with your interviews. You want each piece of B-Roll to be seen and absorbed by the audience so make sure you give each clip enough screen time. For photos and still video shots the standard time is three to four seconds, but with more energetic video you want to show it long enough for the main focal action to be seen and comprehended by the audience. If you are using audio from the clip, such as people talking or loud events taking place, you can have it switch over for a longer period. Often this type of footage is just as important for the story as the interviews so give it some time, but understand that if you are going to give it a significant period of time longer than five to ten seconds it is going to change the rhythm of the interviews you had before it. This is fine if you are planning on shifting gears, but if it is just randomly placed because the footage matches the interview topic then its length will counter its effectiveness as great B-Roll.
Get Great B-Roll
The most important things to remember are not to over use certain types of footage, make sure you place B-Roll according to what the interviews are discussing, and make sure you have a diversity of footage. Do not let one type of image dominate your film because it will begin to feel stale. The best filmmakers jump between photos and live recordings, news reel and stock footage, location videos and old movies. Great documentaries are like a wonderful collage that tell a true story, and the B-Roll is the color that makes it come alive.