What visuals are most appropriate and interesting within the content?
In the past some illustrators have almost prided themselves in illustrating the forgotten parts of a story. The would turn an obscure reference to some subtle emotion into a full-blown illustration. Sometimes they won awards, sometimes they created artwork that they really liked, but sometimes their illustration had little to do with the real story.
An illustrator should choose points in the story or elements from the concept that make the story or concept more meaningful to the audience. What are the major events within the story? How can the concept be broken up into smaller concepts? What emotions? What events best emphasize the emotions?
As an illustrator I try to create the major emotions in the story with the few illustrations that I have. Emotions create a sense of meaning in the audience. The readers can relate better to the stories because the feel them in the depths of their souls.