I think you see it. You explained it fittingly above. You just refuse to call it agency. That’s the rub.
choosing motivations, thoughts and mental states is an exercise of player agency because these things are choices and are consequential to how the character is played and how the character is played is consequential to how the rpg is played.
I think it's more that the context of choosing thoughts, motivations, and mental states, are only
precursors to enacting agency, not agency in and of themselves.
Enacting agency takes place when those thoughts, motivations, and mental states are put into the fiction through the character
choosing to act.
I can see there being some potential confusion, however, around instances where the player says something as if in character that immediately establishes some "truth" about the fiction.
@AbdulAlhazred, I think, brought up something like this earlier with the elf who spent "many a long weekend dipping in the ocean," or something to that effect.
I'm not sure this qualifies as "agency," per se. Yes, we've established something about the fiction (assuming the players and GM just play along and agree that this newly-spoken "reality" is, in fact, "real"). But I think
@AbdulAlhazred's point is that we haven't
meaningfully altered the course of play / course of the fiction (we haven't moved play states).
Sure, we've established something "true" within the fiction, but it has nothing to do with the goal of
playing to find out what happens. It's nice color / texture to the scene and character, but it doesn't have any resonance to the concept of, "What's this game
about?"
Is this game a struggle between downtrodden peasants looking to overthrow an oppressive Lord? A conflict between two long-time friends fighting to "get what's theirs" in the criminal underground? A struggle for a group of rag-tag adventurers hoping to make their next big score in a murky dungeon so they can finally enjoy "the good life"?
My elf character saying, "I used to love going to the beach on weekends" is lovely color, and a good sign from the standpoint of the player engaging with their character---certainly nothing wrong with this, and overall a positive thing. It's just not evidence of player agency with respect to moving the game state.
Now --- if after saying this, the elf character says (through the mediation of the player), "In fact, you know what --- those times at the ocean are the most important things in the world to me. I'm going to go back and do everything possible to get rid of the pirates and corrupt fishing fleets ruining it." And then starts pursuing that as an agenda --- now we're moving toward game state change.
That said, I think that players can make more strongly-worded, in-character declarations that can move toward state change. Suppose, for example, a player goes on and on about their sworn enemy, the Baron von Evilhoffer, describing in great detail some set of past events or feud between them.
Then we start to get nearer to the mark of player agency --- but these are exactly the kinds of things that "traditional" D&D / GMs simply don't care about (literally from
@Lanefan's own mouth --- doesn't care, nor have any interest in engaging in this sort of thing). To bring these kinds of more substantial agenda "pieces" into reality within the fiction, generally takes 1) total buy in from the GM and party, 2) a system that mechanically inserts these statements into the reality of the fiction (Dungeon World has dozens of these), or both.