From an interview with David Carradine in this week’s Onion:
A playwright once told me, “Here’s what you do. You take situations and you build a perfect globe with the story. The problem is that nobody gets to see inside it. So you cut it in half and you show half the story, then let them make up the rest of it.” Because otherwise, it’s opaque. If you tell them everything, they’ll have nothing to think about.
This is in line with some of the best advice I’ve ever gotten from a writing instructor: don’t tell the readers anything they don’t need to know, and don’t tell them before they need to know it.