[Intrepid Homoludens]

Considering that so far we have very, very little information about Heavy Rain other than the claim that the NPC emotions happen in real time, I'm working on speculation, hence my thoughts on it will change the more info we get.

If I'm right it may work a little similarly to Facade, where the characters gauge your decisions, dialogue selections, and actions and react accordingly on the fly (though it might not be as sophisticated as Facade, especially if the game's length, depth, and complexity of plot and interactivity figure in). Given that, it's probably more feasible to think about it not in terms of subtlety, but more in terms of character interaction, depth, and intensity of emotions.