Active and Passive Perception

@JeremyECrawford LMoP, p. 7, if PCs suspect ambush, should DM use passive or active Perception (vs. mob Stealth)?

— John Platte (@jplatte11) July 5, 2015

I usually don't ask for a Wisdom (Perception) check unless a character is actively trying to perceive something. https://t.co/vbw3qgJ39O

— Jeremy Crawford (@JeremyECrawford) July 7, 2015