One thing I like about the 2d20 system is the transactional nature of Momentum and Threat–the way the Narrator is encouraged to spend Threat points to create dangers for the PCs, whether it’s through changing the environment or adding new enemies to an encounter.
But it does raise an interesting question–as the Narrator of a game, I can literally say that anything I want happens at any time. If the characters are walking through downtown Helium and I want to say it starts raining frogs, or that giant war machines burst up through the ground, I can do that.
If you’re running a 2d20 game, how strictly do you hold to the Threat economy?
If an encounter is running super-easy, do you ever add enemies as reinforcements, even if you don’t have any Threat points?
In another type of game, I might decide it would be interesting if the Martian moons both pass out of sight during an encounter, shrouding everything in complete darkness and heightening the tensions. What if I’m out of Threat? Should I just not do that?