To be vague for 'reasons' I'd say it was when I got a response in RP that took me by surprise. We had discussed out the scene ahead of time to some extent but something happened in it that I wasn't expecting though it went with something I would like that really made it far more interesting. It just made the direction of the roleplay that much more engaging and interesting.
Then again I'm sucker for twists and while its risky as heck to NOT discuss those things early, it also has the potential to be very rewarding if its done right.