Perception / Insight rolls
Or more specifically, when to use them versus the PC passive Perception / Insight scores.
When the PCs are actively trying to do something that falls under the purview of these skills (checking for traps, trying to figure out if someone is lying, etc), they roll.
If they are not actively using them (i.e. they come upon a trapped door but do not state that they are searching for traps), then the passive scores should be checked to see if they notice whatever is going on.
An example might better illustrate my point. In the very first scene inside the pub, the module calls for the PCs to make DC 10 Perception checks to notice the bells chiming throughout the city. Unless the PCs state that they are listening or looking for anything out of the ordinary, the passive perception scores should be used. No rolls necessary. If their passive scores beat the DC, they notice. If not, they don't.