Emerikol
Legend
Maybe I overstated a bit. I realize that there is a point no matter how well prepared where the GM may have to improvise on minor details. And as you say there are times where the players will do things like hang around town and see what trouble they can get themselves into when the GM has to make some judgments.My experience is that players do (or ask about) things I didn't anticipate. There are times--my last Saturday session was one--where I look at where the PCs are and I realize I have literally no idea what they'll do or ask, after a few things pending from the previous session (the players are more-than-reasonable about letting me work out answers between sessions, where it fits to do so); in those instances I prep very little--this past Saturday I prepped literally nothing at all (other than answering a couple pending things). I don't think the fact the session was more-improvised than most sessions I run was damaging to anyone's suspension of disbelief. Now, I'm running in a setting I've made up, and I know it pretty well, so--especially since the PCs weren't spoiling for a fight right away--I was able to fall back on that. I wouldn't argue that I'm more knowledgeable about my setting than the players are, but I'd be kinda reluctant to say I'm that much more committed to it.
I may be a bit further along on the prep dial than you but it is a dial. And no matter how well prepped you may have to improv. Most of the time for me this is conversations between NPCs and PCs. I try to inform my improv a lot with good notes on the NPCs and judicious use of die rolls for reactions etc... to maintain fairness.