This can seem like a complicated, or even dishonest, proposition, but it is standard in documentary sections. When you are doing your story editing you take all of your interviews and direct footage and you look to find exactly what elements you want to focus on and will portray the ideas and characters you see for your film. You begin constructing them together to make a story arc, which focuses on specific people and events in the story. This can be a historical or news based story, or one that is more subtle and mostly involves you capturing regular happenings. Either way you may want to include an interview piece as part of the story structure that discusses an item or idea that you do not have any image of. In these cases you may be able to get by without putting any type of visual there, but the likelihood is that if they are saying or doing something significant you will want to pair it with an image.