True, the GM has to be amenable to such changes and it helps if he or she provides alternate avenues of plot advancement but a GM can only do so much.

I once had a group that decided, instead of going to the King's castle to answer the call for heroes, elected to get jobs in town... Yes, they did. I had the Kitchen Help, Barmaid, Stableboy and Blacksmith's Apprentice. I just about strangled them. It's a lot more work to bring the adventure to such "heroes" on short notice :) NOW, if they told me ahead of time that they were going to be "ass hats", maybe by mentioning in their backgrounds that they weren't the adventuring sorts, I'd have come up with a murder mystery suitable to such a venue.

I think my table became much more interesting when I started putting in some crazy rules... like putting up their hand to speak out of character. Being human beings they were too lazy to lift their arms all the time so they got more used to talking in character :)