I model people constantly, but agency and the "PC vs. NPC" distinction don't even come into it. There are classes of models, but they're more like classes of computational automata: less or more complex, roughly scaling with the scope of my interactions with a person. For instance, it's usually fine to model a grocery store cashier as a nondeterministic finite state machine; handing over groceries and paying are simple enough interactions that an NFSM suffices. Of course the cashier has just as much agency and free will as I do -- but there's a point of diminishing returns on how much effort I invest into forming a more comprehensive model, and since my time and willpower are limited, I prefer to spend that effort on people I spend more time with. Agency is always present in every model, but whether it affects the predictions a given model outputs depends on the complexity of the model.