My issue here is not that there may be hidden details of a NPC.....that's fine, in and of itself. My issue is when that is the case, but there is no means for me to learn those details, and then the knowledge of those details would be key to resolution. It's kind of a catch 22, right?
In my experience, a GM far too often holds fidelity to these pre-established facts as too high of a play priority.
Sure, that's all fine. Again, my dissatisfaction in this area is when this is all left up to the GM and I as a player cannot act on it without the GM basically deciding "okay, that seems like a reasonable approach". If actions such as Intimidation and Persuasion and Diplomacy and Bluffing are in play, then I'd like to be able to use one or all of those in some manner that I as a player can understand.
Because ultimately, the GM's judgment isn't something I can predict. And I know your view is that "well in real life you wouldn't be able to know what another person is thinking" and yes, that's true. But in real life, I'd likely be able to bring a lot more resources to bear other than what a GM tells me.
The player's understanding of the world relies on what the GM can share. That is, by nature, going to be incomplete. For me, rules and play processes can help bridge that gap.