So a player best have a very complete backstory lest they leave out details that could cause conflict of player vs PC knowledge during gameplay? Seems like a high expectation for players, in that case.
Luckily, most of the obvious stuff is covered by background choice, skill proficiencies, etc. Something I failed to mention before is often if something is in doubt I have the player roll an Intelligence (or whatever) check to determine if a random piece of information is available to the PC. Honestly, this sort of thing rarely comes up...
The larger issues which seems to have been washed over is the metagaming issue when player knowledge deals with things that the PC is not involved in or aware of, etc.
So, in your games, the DM knows the PC better than the player?
In some ways, yes. The reason is
mostly concerning INT scores. But that is another can of worms that is probably
also left for another thread entirely...
Another reason is I know my
game world better than the players because I made it. They are part of it, yes. And FWIW, there are many times when I do the
exact opposite and clue my players into something their PC
would know but that they, as a player, are not aware of.
It works in both directions, something people also don't seem to realize (mostly because they want to jump on the "that is not fun" bandwagon...).
Some might argue that you cannot reasonably separate player knowledge from PC knowledge 100%. But I think that is fodder for another thread entirely.
Well, if I am not hitting 100%, I sure as hell try my hardest! I'll admit it is hard, but it can be done.
That's just the thing that many DMs here have given up on: policing how the PC thinks, talks, and acts. According to the 5e rules (or, if one prefers, guidelines), deciding how the PC thinks/talks/acts is the player's duty, not mine as DM. If I ever ask for clarification about WHY a PC is saying/thinking/trying to do something, it would be out of simple curiosity of wondering what they are hoping to accomplish and to possibly hear something from the player that might add to the fun, memorable story. It is never to tell the player: "Sorry, that's not how your character would act/talk/think." A fictional reason is readily available for any of these - and I, as DM, don't want that extra duty so I leave it to the players. Sometimes the players choices are optimal, sometimes they are suboptimal, sometimes (either way) they can earn Inspiration.
Policing is only necessary in very certain situations, and honestly pretty rare, and 90% of the time due to unknowingly metagaming.
If other DMs want to allow players to use player knowledge their PCs would not (likely) have, use roll results and knowing outcomes of success/failure and other instance to metagame, that is their prerogative, certainly, but not something I would do.
And I think several people here think this "challenge" from the DM is one that gets in the way of smooth game flow. I know from past experience of having run this way. Once I dropped worrying about player motivations behind PC actions, our 5e game became much, much more enjoyable for the players at our table and, especially, for me as DM. Clearly, YMMV.
My enjoyment is also part of the equation since I play the game as well as my players. Obviously I have no idea how disruptive you found such things in your experience in the past--but I don't find it disruptive at all and as I said in an earlier post, when it does come up and I have to adjudicate it, the response is nearly always, "Oh, ok, I'll do ___________ instead."
Also, I would like to express my appreciation for your post and your non-judgmental manner. Thank you.