In my D&D game, I'm unsure whether to allow players to use their Passive Insight or Passive Perception instead of rolling when they are actively trying to inspect something. Do I make them roll when they're actively trying to inspect something or someone, or do I allow them to use their Passives?
Let's start with the basics:
At it's most basic, passive perception/insight is the default state for PCs. This should inform you about how much information to give right off hand, and what to keep secret. If they want to know more than their passive scores tell them, they have to roll.
To add to this, the keyword is passive that means that this is something the DM just implements. This is a measure of how closely the PC is always watching and thus it's up to the DM to say, yeah you notice X because your passive skill told you. However, if a PC wants to actively do something, it's always rolled.