A two-column script is an easy way to structure and plan for your documentary project. Even if you don't know what your interviewees will say or what footage you might shoot, writing a script predicting what you want your final film to look like is a great way to stay organized. Once you do shoot your documentary, you can update your two-column script to reflect what video and audio you actually have to work with.
In the left column of your script you will describe all the VIDEO your viewer will see (b-roll, a-roll, photos, diagrams, screen captures, etc.). In the right column, you will describe all the AUDIO the viewer will hear (interview bits, music, narration, sound effects, etc.). Think of each row as one scene in your documentary, with the audio playing over the video.