So how could a characters emotion be best expressed in a roleplaying game? Normally this is just a matter for acting or storytelling. For some point buy systems, characters might pick up phobias or love-interests to protect, but even then things are often kept fairly mechanics light. That’s fine for a certain style of game, but trying to manipulate other peoples emotions is ultimately part of the tactics of social conflict.
Reading over Mike Mearls’ blog, he had an post on adding some flavor to NPCs:
I've been thinking of doing something similar for my NPCs in 4e. Sure, they'll have the typical spells and tricks to make them daunting enemies, but I'd also like to insert a few powers that are a mechanical expression of the NPC's personality and role in the campaign, built along with quotes or other material to go along with the attack…
I like the idea of these "scripts" because they make a fight different. Sometimes, the tactically smart play for an NPC is boring and flavorless. If every NPC fights as well as the DM can run them, you lose a lot of what makes an NPC unique. Ideally, the players think of the fight in terms of the NPC's personality ("That dwarf was crazy! He ran across a pool of acid to get to Baldar.") rather than in terms of powers ("That dwarf had a nasty sneak attack abililty.")
The specific examples he gave were all rather combat oriented but got into confidence, love, anger, and hatred. Mearls was talking about special powers to emulate these feelings, but in some ways they could work just like being slowed, blinded, or stunned. This isn’t to say that powers couldn’t key off emotional statuses, barbarian rage is a classic fantasy trope and could fit well with such a system.
The first question I’m pondering is whether emotional statuses should be fairly neutral or negative? Hatred might give you penalties when attacking someone other than your rival, but should there be a bonus for hitting the rival? Anger might make you more prone to leave yourself open and thus give a defensive penalty, but does it make your combat or social attacks more biting? I’m prone to think a mix of advantages and disadvantages is best, but that makes the states more complicated.
Here I think I’d like to do more research. Social mechanics should ultimately be a bit more real than combat mechanics. It’s an area where most of us have more real life experience, so it’s easier to get it wrong.