Anyway, we've all read fantasy novels where a stranger comes to town and this stranger knows a lot about our main character. Thinks the main character doesn't know, and things the stranger doesn't want to tell him or her. (J.K. Rowling escapes this one early on, but falls victim later. You know, at least Hagrid says "You're a wizard, Harry." But later, Dumbledore keeps plenty of secrets from Harry.)
Authors use this without thinking, I believe. It's important to the stories they've structured that the vital piece of information isn't revealed until the author is ready to reveal said information. This means that someone who might be in possession of this information, won't tell the person it affects. Some examples:
- Moiraine in the early Wheel of Time books
- The Sheason (sorry, I forget his name) in The Unremembered (which was, as you may recall, a pretty blatant rip-off of the Wheel of Time books)
- Obi-Wan Kenobi knowing who Darth Vader is/was and not letting on to Luke (he may have planned to say something later but of course he got killed)
Here is a relevant TV tropes page, though it's not exactly what I'm thinking of: plot driving secret
Anyway, the point of this whole entry is: if you're writing a fantasy novel, don't do this. It's irritating and the readers can see right past you.