Yesterday, I was doing a last read through of a revision and I ran into a scene in which a character reveals a secret to my narrator and my narrator reveals that he already knew the secret. The narrator's reaction is a simple "I know," and I debated whether I needed to include an internal narration that would point the reader to all the clues that had been obvious to my narrator.
My gut told me no. I'd left enough clues throughout the story and I had to trust that the reader had picked up on them and had guessed the secret right along with my narrator. Adding that internalization would have cheapened the moment and blunted the impact.
But it's so damned hard to trust an audience that I don't even know. I know that there are going to be people who read it who will not have picked up on the clues, and some who will feel as if I've made it too obvious. If I don't trust the reader, though, then I end up with a crappy story.
Trusting the reader is one of the most difficult things for an author to do. It leaves so much up to chance, up to the mercy of the person holding the book, whose experiences may not match the author's own. But it's something every writer must learn to do. We can't spoon feed readers, nor should we. It's bad writing and offensive to readers everywhere.
Take the leap of faith with the readers. Your book will be stronger for it.