[Rudar Dimble]

But do you have something like a script ready for what happens if the good guys do nothing?

I usually do this:
1. make up beginning
2. make up possible endings
3. create NPC's
4. connect various NPC's to eachother and decide what goals they have and how they want to achieve them
5. create a few cool encounters for the PC's
6. add some side-quests

So basically I only have the beginning, a few possibe endings, NPC's (in detail), some (important or cool) encounters related to the main quest, and some side-quest.

All other (minor) encounters and events are improvised.