There's been a lot of experimentation within games lately to try and have methods for interactive dialog while maintaining the flow of things - trying to break away from the complex dialog tree where you have to read every response and pick the most appropriate. (Don't get me wrong, I still like dialog trees.)
Some of them, I think, are trying to shorten things to a simple 'icon' approach, where you choose a positive/negative/humorous/angry response, without knowing the details, and then the character delivers whatever line the developer wrote. But while this may get you moving along faster, it still breaks flow.
And particularly in terms of modern 3d-roaming games (which I generally don't play) there's this awkwardness to standing frozen in one place staring at each other passing the dialog puck back and forth.
So, I was thinking - and Heavy Rain *may* have already done this but I don't know because I didn't play it - could it work to have character emotions controlled through body posture, through joystick manipulation? And done *as the conversation goes on* rather than at choice points?
So, while someone's talking to you, you can make your character wobble her head back and forth and look away from the speaker to indicate boredom. And as the dialog moves along to the next branching point, it checks to see what the current emotional state of the player is, and if there IS an appropriate reaction on file, shifts tracks to that part of the dialog. "Sorry, am I boring you? Let me sum up." - or another character might just yell "PAY ATTENTION!" before going on with what they were saying.
Similarly, making your character slouch and sigh could indicate sadness. That could make the person you're talking to express sympathy, or it could just change the method in which the PC delivers information. Talking about a mission that you've just completed [SAD] is different from a mission you've just completed [ANGRY].