Will it be possible to be VERY careful about who sees you do the quest for the gaurd and therefore not effect your relationship with a faction?
Highly doubtful considering there's no way for an NPC to tell which path you took. (ie whether you cross in front of thieves or not when doing a guard quest is not recorded.)
I would have to say that in terms of "how does everyone in the faction know" that they would talk to each other - like through a thieves' guild or simply what they call a "ghost-to-ghost" communication where each member speaks to only two others - or the fact that it's a generalization because it's a game.
(NOTE: What I said in this paragraph is a suggested explanation and in no way indicative of the opinions of other devs, the settings department, or what may or may not be the actual official explanation.)
Idoru, when you're talking about "informants" and such, there are in fact quests with different endings that will give you different faction changes (once it's implemented,) and more are coming. Not saying one exists or will exist exactly like this, but for example you might have a quest that either improves your standing with guards or with both thieves and "chaos", depending on whether you follow the quest as directed by the quest giver or deviate.
