Tales From the Yellow City
Simply put, PVP is not allowed. It is always easy to think of a reason why your PC will attack another PC, and frankly that is boring. It is more interesting to think of a reason why your character can’t attack another even though she wants to.
On a related note, it is assumed that the PCs know each other well enough to not mind working together. This does not mean that they will never work against each other, but such conflicts should be something that both players agree to weave together as friends.