>>535355326
If you think about it for a second, Stillness of Mind should indeed be a passive, not a reactive ability. Your Monk is charmed or frightened. Because they have undergone rigorous mental training, they can instinctively use a moment to end the effect - even though the metagamer controlling them might want to use the action for something else. Rules are not set in stone because the PHB says so. 5E allows DM flexibility and encourages homebrew rules. Larian decided natural 1s and natural 20s would be critical failures and critical successes for skill checks since it's more fun that way, even if not "intended."