I’ve started DMing a one-on-one D&D 5e adventure. The PC took the Observant feat and as a result, has really high Passive Perception and Investigation (20 and 18 at level 1!). If I use the rules from this question (“if passive perception is higher than the DC, the PC doesn’t have to make a roll to succeed”), then essentially every single perception and investigation check in the scenario I’m running is an automatic success.
In a group setting, that’d be OK: this makes the PC better at scouting, which is rewarding and fun, and it’s generally nice to be the only/first one in the group noticing things. In a solo adventure, however, I’m afraid that it’s going to be somewhat boring. There’s less of a “wow” effect to noticing small, hidden details when there aren’t people around to be impressed by it.
Should I just stop worrying about it, and simply be OK with my player basically automatically succeeding in every perception/investigation check without thinking twice about it? Or is there a way to tweak the mechanics somehow to make it cooler or more interesting?