Last session Rob chose to leave his character in a incriminating situation - a city official had decided that the group was a threat to him, and had screamed for help.  The rest of the gang fled the scene and Gaald stayed.  And was arrested and hauled off to the pokey.  I had not planned for any of this to happen. 

I love shit like that. 

When I sit down and plan out a session, the only things that I have firmly planned out are the combat encounters, the information that I want to give the players in non combat encounters, and some sample NPCs to impart the info.  I dislike the DM's Guide method of playing out non combat challenges, so I tend to just wing it.  Skill checks where they seem called for - more checks if the person is not inclined to help.  I also tend to punish excessive "metagaming" where the players treat NPCs like information slot machines. If the players say something stupid to an NPC that would make a person annoyed, the NPC gets annoyed.  For example, back when they were interviewing the old smith from the werewolf village, Gaald tried to tip him for his help.  This was a dumb move - the man was an established, successful citizen, who had lost most of his family and friends in the wolf attacks.  He was recounting a painful personal memory, and the offer of money was insulting.  I had him stand up and leave and the players never got the rest of the info he might have given them.

I do my best to make conversations with NPCs go like conversations with real people.  I try to give each NPC motivations and reactions commensurate with their personal goals and concerns.  An example from our last session:  Gaald, having been arrested, tried to convince the duty sergeant that he should be let go, because the group was seeking an artifact of evil power in the sewers.  This interested the sergeant greatly, since the city guard was about to go on a raid into the sewers to drive out and arrest the criminal element that lived there.  But he didn't know Gaald, didn't trust him, and so didn't let him go.  Instead he reported to his superiors and an arrest warrent was issued for the rest of the group.  It was illogical that the city officials wouldn't treat them as a threat, what with their party showing up in the city and talking about the sewers just as a major purge was about to happen.  This has considerably complicated matters for them, if they still want to get to the artifact before the city forces do. 

And I had no plans for any of this.  All I had in my head was a bunch of NPC templates, and a general idea of how the power structure in the city worked.  The PCs were in charge of what happened and when.  This sort of play means that I have to adlib and invent characters on the spot and come up with consequences for actions that I could not have forseen.  It means that I'm scrambling at times to keep up with the PCs and the crazy shit that they pull.  It means that I get to have fun too. 

It also means that my PCs are going to do dumb things now and then, because they're all video gamers and have been trained to expect limited options and railroad plots.  Rob's tendancy to tip everyone comes out of the concept of faction, I'm pretty sure.  This was only our 6th session, but I expect that everyone will get the concept pretty fast - treat NPCs like real people, because that's how they'll be treating you.  This doesn't remove the possibility of deus ex machina if they get themselves into a complete mess, but I'm going to be working really hard to make it seem at least a logical progression from prior events.