Reading is often seen as a passive act: we lie back and let writers pipe joy into our brains. But this is wrong. ... When we read stories, this massive creative effort is going on all the time, chugging away beneath our awareness.Gottschall is referring to the fact that we fill in so many details ourselves when reading. We know how a character's face should look, the details of their clothing, the surroundings of the action in a way that the writers haven't described.
Jonathan Gottschall, The Storytelling Animal
It is certainly one of the reasons I tend to avoid movies made from books I love. They never get those things right. How could they? They weren't in my head when I learned to love the book.