Well, actually I kind of *do* mind it. Once a player has an idea of what is acceptable and what isn't, that ought to be good enough, ne?

And the surprizes you're talking about are more on the order of 'plot twists'. I see no problem with a character being mysterious and unknown. That's the joy of a new book - you're discovering the character as you read along. When you lay everything out beforehand, it takes something away from that... If you *want* to reveal those details, fine. I just don't think you should *have* to.