So fear and stress apparently work like this...
Seeds of Fear. Characters have up to three Seeds of Fear, specific things that they're afraid of. There's a d12 list, but you can make up your own. When a player RPs their character responding appropriately to their Seeds of Fear, they gain Inspiration. But no more than 1 per 24 hours. That's it. So if they already have Inspiration, there's no bonus for playing along.
Fear. DC15 Wisdom save or be frightened until the end of your next turn. Various reasons for having to make that check, including encountering a Seed of Fear.
Stress. Characters have a Stress Score that goes up and down based on stressful/relaxing situations. From 0 to whatever. This Stress Score is applied as a penalty to any ability checks, attacks, and saving throws.