- Example 1: The player decides to use a glaive as a weapon - an unusual weapon in that campaign world.
This isn't illusionism. When the player decides their character uses a glaive and the umpire doesn't "sure check" them, they're both deciding that glaive-wielders and magical glaives are going to be a little more common in the campaign world.
If I put a magical glaive in the hands of a minor villain for the player to covet and plunder, that's just encounter design. If the player decides to avoid fighting that villain, they don't get that magical glaive, until they run into that villain again or another minor villain that I've armed with a different magical glaive.
Admittedly, I run a heavy improv style where I don't establish a lot of facts until the PCs request evidence that confirms them. But
illusionism isn't designing encounters and adventure scenarios around player interests and character goals; illusionism is changing the established facts of the game world in order to have the players' choices inevitably end up where the umpire wants them.
If I'm running a prepackaged adventure with a villain who uses a three-handed karelian chickentickler, I might very well switch it out for a songhealer's double chainsaw. The difference is, that's a decision I'm going to make before we sit down at the table to play.
- Example 2: The DM has a pre published module they intend to run. That module will include various encounters but it starts with a tavern brawl. This encounter doesn’t need to be keyed to any particular tavern but without it the PCs won’t be part of that module. The DM has that brawl take place in whichever next tavern the PCs visit. Should the GM skip this section because the party don’t elect to stop at a specific tavern?
Look, pre-published modules are
full of things I consider poor form on the part of an umpire and I think that might be simply a creative limitation of the nature of the product. But if it's hinged on a tavern brawl breaking out in any one of the taverns available to the characters... it breaks out in the one the PCs are in. Maybe a very similar tavern brawl broke out in all of them near-simultaneously, which isn't that unlikely. Maybe it breaks out in the ones the PCs are drinking
specifically because the instigator followed them, specifically, and started the fight with them.
But if the PCs decide to break up the fight without participating in it, or they decide it'll be cheaper to share an unused stall at the stables... then yeah, the umpire should either look the players in the eye and say
choo choo or they should improvise another way to get the PCs involved in the rest of the adventure. Asking the players to
buy in, explicitly and OOC, is a small spend of the umpire's authority that
for its explicitness comes across as an exception to the rule that only makes the players' free choices feel more authentic to them.