The way this game would work, there would be a few main storylines (multiple factions) and a few side quests that can be proposed or invented as I go along. Players would be able to control NPCs related to their own character, but the majority of world events would be dictated by myself, in a style I would feel would be realistic. 

Telling a guy you don't know to fuck off, he appears later as someone you need to work with, he's not so friendly because of earlier, etc etc. 

I'd really stress players to look at all their options, even if I don't [i]present[/i] all of them. Using the example above, you could either try to sweet talk your way in, bribe the guard, kill him, etc. Or, look for alternate ways in. You climb up a pipe to a window, but the window is locked from the inside. You could either try to quietly break in, or look for another alternate route. 

That's just in terms of how the game is played, I suppose. The above advice of making a cohesive group is much appreciated.