NPCs that continue
Most game NPCs exist only while observed. They stand at a post until a player arrives, then return to it. Nothing happens to them in between, and players notice.
Project Echo investigates NPCs with routines, occupations and relationships that continue independently of whether anyone is watching.
The cost of a living world
Simulating hundreds of characters continuously is not affordable at full fidelity, so the research centres on level-of-detail for behaviour rather than for geometry.
Characters near a player simulate in detail; distant ones advance on a coarse model that produces plausible outcomes without step-by-step simulation, and reconcile when a player arrives.
Memory and society
Long-term memory lets a character remember a player across sessions and adjust accordingly, which is what makes the world feel like it acknowledges you.
Characters also interacting with each other is where genuine emergence appears, and also where the system is most likely to produce nonsense. Bounding that is the core difficulty.