Social mechanics derailing my plot

I was sitting here thinking, while writing my previous post, that one of the things that bothered me about Dogs in the Vineyard, and any game with strong social mechanics, that I don't like the game mechanics to tell me how my NPCs act or feel.

And it struck me… is that really any different than the mechanics telling me that the PCs took out an important villain two scenes before I expected them to? Is saying, "Despite all your interrogation efforts, the spy doesn't tell you anything," any less of a railroad than, "Despite all your efforts to stop him, the assassin escapes into the night"?

In the latter case, the players are going to expect a fair chance to stop the assassin. No matter how much I want the assassin to escape to further my plot, the players are going to get surly if the assassin escapes by GM fiat.

Should it be any different in the former case? If the spy revealing what he knows messes up my plot as badly as the assassin being captured, do I have some greater reason to control the outcome in a social conflict than in a physical, just because it has to do with mental decisions and not physical action?

Should I let social mechanics drive my NPCs' actions? (Presume for now that the dice aren't stupid and never make the character act out of line with his personality and motivations.)

One one level, I can see where physical mechanics are meant to represent a whole lot of things, including the randomness of a situation. But how much is represented in the workings of the mind? Do I break under the pressure? Well, my mind can't slip in the mud or be blinded by a flash of light, but are there other factors that justify using mechanics?

Here's an interesting thought… PC's use Fasttalk skill on NPCs, but how often do NPCs use Fasttalk against the PCs? How many players would have a cow if the GM said, "The assassin uses his Fasttalk skill against you, and rolling a Superb result, manages to confuse you long enough to get through the door you're guarding"? Traditional social mechanics have always been, in my experience, one-way… used to simulate the PCs' abilities of persuasion, etc. against minor NPCs. And when you get to the big-bad, you don't just roll against your Fasttalk to convince him not to blow up the world, you have to roleplay it out, and the big-bad's decisions are made by the GM.

I think the big issue is the feeling that the mind is totally under our control, and we should get to make our own decisions and not be subject to the dice. Roleplaying games are very often about making good decisions… and to have the power to make our own decisions about what our characters do, PC or NPC, just feels wrong.

This is certainly something for me to chew on.