I love being in the middle of reading something and being handed things to figure out, imagine, block, and dialogue for myself, thoughts to think for all the other characters, that kind of thing.
Oooh! Me too! Me too! *bounces* And it's something I love doing when I write (how successful I am is of course something else entirely, but I love the concept) - letting characters communicate wordlessly or dropping clues to later (or earlier) happenings that might only be revealed many chapters further on. Having things happen in the background, or off-screen, that shows the world is bigger than just what we see. Bascially I love layers, which is probably why I'll never tire of Buffy. :)