Correct. If it gives an advantage you can't select it; and the random roll aspect reflects the reality of some people just being born luckier than others.
If it was decided up front that all the PCs would have some sort of advantage via their backgrounds, that's a fine table rule. The reality is, though, that the vast majority of people in ye olde typical medieval society were peasants who really didn't have much going for them at all; and I don't mind if the game reflects this at least to some extent.
Sorry to take so long to reply....I had a busy weekend.
I think the bit I quoted here is really just about preference. I know based on our past discussions that you prefer to play with the expectation that the PCs are "no one special", they're just another person in their world. Which is fine, of course. I don't really care to try and hew to some kind of quasi-medieval social class expectations; my players come up with characters they want to play, and I work with them to make that happen. Any advantage they get from their background is easily offset with an equal disadvantage.
I also think that the life of an adventurer is simply different from whatever society woudl consider "normal", so to me, PCs are special no matter what. They don't need to be some kind of "chosen one" or anything, but I don't think that they are typical by any means.
But again, this is simply preference.
It's different because when being decided on the fly it's usually being decided for a reason: the player/PC needs or wants (and thus is asking for) an advantage in the here and now.
Chances are that had the player/PC known about the advantage ahead of time, the roleplay leading up to this point would have been somewhat different.
Example: party arrives at Karnos, an unfamiliar and not-that-friendly town. Player A, who has up to now left her character background mostly blank, suddenly declares "Oh, don't worry - I'm the local noble here and my word is the law. Everyone knows me. And look, here come some of my personal guards now - they saw us coming.".
If this (that a party member is the local noble here) had been known from square one the party's dealing with and feelings toward Karnos would have almost certainly been much different. Very likely they'd have used it as a safe home base all along, rather than only coming here now because they have to.
In fairness, it's always possible that the player for some reason had kept her PC's noble status a secret up to now; but that's a different matter.
Aren't many of the choices made by players for their characters made to gain an advantage? Weapon or ability selection, spell choice, feats versus stat increses, what magic item to wear in their belt slot.....all these things are done with advantage in mind. There may be other factors as well, but mechanical advantage is likely always a consideration.
Why is that a problem in the scenario you describe? I will point out I think it's a bit of an extreme, and certainly different than the one I presented in a couple of key ways, but still it may be interesting to discuss. What's the big deal if the player does decide to claim lordship of Karsos? Sure, it may make things easier for them in the immediate "hey the guards aren't gonna kill us" kind of way, but I woudl also think it would open up several opportunities. What's the PC's place in Korsos? Are people happy for him to turn back up? Was his family glad he was gone? All kinds of political angles seem to present themselves.
Now, if the goal of play is not to get embroiled in the political situation in Karsos, these concerns don't need to be raised. Perhaps something else can be done with this bit of info. But the question is if this isn't the goal....if this isn't what the player wants, then why would they introduce this idea? Just to avoid being bothered by some guards in a potentially hostile town? Seems a bit of a big card to play for that reason.
Does this interfere with the DM's plans? Or the other players? If so, can that be reconciled? I would imagine a conversation would happen, and the best way to proceed would be decided on by all.
Right - back at it...sorry 'bout the gap there...

Because in session 0 there's no here-and-now stakes, and no clear and obvious immediate advantage to the PC/player. In session 4 when the stakes have become serious it's a bit beyond the pale if Tommy pulls the answer out of thin air like that.
And even then it's probably not the end of the world, except that if Tommy does this once what's to stop him doing a similar thing - that his PC just happens to have the answer to a situation or puzzle or whatever - again, every time his PC is stuck but he-as-player knows the answer? And the answer is, of course, nothing; because the precedent has already been set by the DM allowing it to happen in session 4. Pretty short hop from there to outright bad-faith play.
Again, I don't see the problem with the stakes. To me, it's the idea of the player's background actually becoming important in play. That means the player will likely be more invested because the character is more tied to things.
As for the "slippery slope" kind of argument....I don't think that's really a concern. Perhaps with certain players or certain groups, but I think that in general most players can actually handle this without abusing it. It may take a little adjustment to actually incorporate this kind of thing into a game where it previously didn't exist, but I think it's achievable.
Any of those is possible, sure, given the right situation (e.g. screaming or shouting a warning is only any use if the rest of the party is still within earshot; the PCs reading signs assumes they are following and not staying put so the scout can find them when she returns). The "hunch" one is valid, but would get contrived if done too often.
Well, in the case of a wizard or cleric, I don't know. In my 5E game, one of the characters is a Diviner. She gets those kinds of hunches all the time. Perfectly within the fiction that's been established.
And I'm sure we could come up with an explanation for just about any scenario.
The easiest would be to not confirm that the PC is actually dead. Just cut away leaving her actual status unknown. Maybe she's in negative HP, or making death saves or whatever. Then you'll actually get honest action from the players. This would probably be ideal if you want to avoid metagaming.
Oh, absolutely.
It's when players start talking in-character about their late companion and how they need to go back to town and find a replacement when they don't and can't even know she's dead yet and she's not due to return for another hour or so...that's when the smackdown hammer comes out.
(even worse is while she's still alive and out scouting other players won't just shut up and let her player play it out, they insist on offering suggestions or even telling her what to do when their PCs have no way of knowing any specifics of the situation at hand)
No. I ask why are they suddenly moving now when they'd agreed to wait here for at least an hour for her to get back - are they intending to abandon her? And if the answer comes back "well, she's dead" then someone's probably about to get yelled at.
Don't you just flash forward past the hour of waiting? I would expect so. "Okay, an hour's passed and the scout has not returned....you all have an uneasy feeling about this," and you're all set. Play proceeds largely as it would have without the need for pretending not to know what we know thing. The players can play their characters without their knowledge of the scout's death impacting their decision making.
Sometimes I think the attempt to avoid metagaming involves more metagaming than what is trying to be avoided.