This is decidedly a hairy topic for me to delve in. In role-playing games, I've noticed that social interaction is typically free form. Naturally, a player may assume that introducing mechanics to interaction is essentially non-interactive. I don't want to railroad, however. I want to offer new and exciting avenues of approaching a staple of role-playing.
Sometimes, in a session, players will go in circles trying to gain the upper hand in social interaction or attempting to derive information. In one session, I observed players interrogating a crooked merchant for HOURS. Sure, the role-playing was good, but it was fucking boring, and two of the action-type players were effectively removed from the session.
In Dungeons and Dragons, balance of interaction between the DM and the players is offered via the charisma attribute and the related skills bluff, intimidate, and diplomacy. Without it, no matter how persuasive or clever a player character is, a DM can be as inflexible with his NPCs' responses as he wants. Thus, railroading and circular interaction. However, I feel that these mechanics are...well, mechanical, because a character with many points in these skills may circumvent engaging interaction (Gamist design attributes superimposed over a Narrativist aspect), and serve to undermine the required depth of social interaction. This is why I believe players choose not to invoke these mechanics.
I offer mechanics not to force results from NPCs, but rather to give insights to the player as to how he may skillfully navigate social situations. My proposed method is to combine the effects-based techniques system with the personality and philosophical profiles presented in my previous post.
For example, a PC wants to coerce an NPC into doing something. So, he uses a technique to discern the NPC's philosophical profile. The GM compares the player's Insight attribute to the NPC's ability to hide his true thoughts and feelings (Charisma). The player passes with flying colors, and the GM reveals to the player that the NPC has an extremely strong bias towards Principles, and since he passed with such a degree of success, the GM will tell the player what those principles are. A resourceful player may then use this information somehow to persuade the NPC that non-compliance will violate his principles. In this instance, GM inflexibility will be noticed, and thus deterred. Another way it could be used is to discern the NPC's profile, and then pretend to act in a similar manner. Because of common ground, the NPC will be more inclined to like or trust you, in which your Charisma adjusts the magnitude.
As a byproduct of this system, I think that these dynamics will bring out vivid, complex NPC personalities and provide for a very organic experience of social interaction.