Shot list first, then over-shoot on purpose
'One of the worst things that can happen to you after filming a video is to get into the editing process and realize you just don't have enough B roll.'
The script becomes an itemised shot list, and the shot list goes to the client before the day so that ideas which are not realistic get killed while killing them is free. On the day, coverage means two different things and both are required: shoot many different subjects, and shoot the same subject from several angles and perspectives, which is what gives the edit depth rather than just length. The instruction that turns this into a habit is to film everything at least twice - once covered the standard way, then once the way you think nobody else would shoot it. What is being bought here is options in the edit, and options cannot be bought later.
A coverage gap is discovered at the only point in the process where it cannot be fixed.
B-roll is filler you shoot to pad the timeline out to length.
It is what buys you options - cutting around a stumble, changing pacing, restructuring the story after the fact. Options you did not shoot cannot be bought in the edit.
One of the worst things that can happen to you after filming a video is to get into the editing process and realize you just don't have enough B roll to adequately fill up your timeline.