It’s always necessary to remember that the fiction writer is much less immediately concerned with grand ideas and bristling emotions than he is in putting list slippers on clerks.
Mystery and Manners p. 70
Her comment is in relation to details an author includes that have little to do with the plot but that make the world seem real: here the clerk walking down the street was wearing list slippers. Tolkien was certainly the master of this: creating language, culture, history—some of which never showed up in the actual story, but which gave Middle Earth solid bones on which to build a grand story. Without the details that make a place come alive, no amount of grand ideas will make a grand story.
Lingering Over Leaves 