I answered something similar to that one before:
http://www.zelaron.com/forums/showth...threadid=19108
Set the switch at the end of your quest, and on every character you want to react, add a page to that character's event with the precondition of your switch. (Read above for more about switches)
Basically, if Weak Town is getting ravaged by a monster in Blah Cave, and you fight the boss in Blah Cave, the boss event's last line on page 1 (or wherever the fighting page is) should be turning on a switch (I'll call it 0082:KilledBlahBoss). The boss's last page should be blank, with the 0082:KilledBlahBoss switch as a precondition. All the characters in Weak Town should have a second page as well - with 0082:KilledBlahBoss as the precondition.
Simplified:
Step 1: Choose a switch and name it.
Step 2: Set all the townspeople you want to be affected by the switch, and add a page with the switch as the precondition.
Step 3: Make the townspeople react approprately. Maybe a few "Good job, HeroGuy!" type messages.
Step 4: Set up the quest events. (If the events are only to happen before the quest is finished, set up a second page with your switch as its precondition... an example would be a loud ROAR every time you enter the cave.)
Step 5: At the last step in your quest, turn the switch you chose ON. This will disable the quest events and make the townspeople talk to you as though you just did something (using the Page 2 dialog)