Let's face it. There are enough complications to face without having to get into conversations about these items with other people. Let me suggest 3 sources of motivation. A city is under siege by enemies. Attacking army keeps everyone from going out or coming in.
Supplies dry up.
People begin to starve. Two lepers live outside of town, between the city walls and the besieging army. Eventually they get so desperate that they head to the army tents. We'll die here, anyway. When they get to the tents they discover that everyone has fled.