No, it's different. The second level fighter Jezal being played by 38 year old player Tony, who has been playing D&D for 30 years, may actually know that Trolls are vulnerable to fire. Unless Tony and the DM have a comprehensive list of all the things Jezal knows and a list of what he doesn't know, then it's actually a gray area at the time the troll shows up in the game.
So the DM decides on the spot that Jezal does not know about trolls. This is the DM removing options from the player.
It's only a grey area if it's possible for Jezal to know about trolls. Suppose Jezal grew up in the middle of a desert in a secret oasis where nobody left for fear of discovery. He was the first to get away. Trolls would be unknown in a place like that. There would be zero chance of troll knowledge being available to him. By ruling that he does not know about trolls, no options were removed from the player since that option was never available in the first place.
There are many backgrounds where troll knowledge wouldn't be available. Many where it would be grey and a roll would be required. And many where such knowledge would be automatic and Jezal would just know. This entire game is designed to be played that way. It's not up to the player whether or not the character knows about trolls. It's up to the DM to rule whether such knowledge is uncertain or not, and if it is, to allow the roll.
Sure. And there are times when it's very clear what the character knows or doesn't know...and there are other times where it is not so clear, correct?
Absolutely. During those times a roll will happen. That's the rule. Knowledge is a skill. When the results of a skill are uncertain, you get a roll.
Do you see the DM's decision to determine on the spot that Jezal does not know about trolls as limiting Tony's choices as a player? Tony hadn't made a decision what to do yet...we're just looking at the choices available to him. Has the DM limited those choices?
No. If it certain that Jezal has no knowledge of trolls, then Tony never had the option to have that knowledge. It would be no different than my deciding that a wizard could not cast cleric spells. The option was never there, even if Tony does know the names of all the cleric spells and what they do.
Immersion can be fun, sure. But it isn't a guarantee of fun. Hence my example of a boring encounter with nothing going for it but the "mystery" of how to hurt trolls.
It really depends on the person. For a lot of us immersion is fun in and of itself. If an encounter is fun or not fun, that's separate from immersion.
So, given the choice between immersion and fun, I would say fun is the more important element to maintain.
Again, though, this is for you, not for everyone. The group I play with values immersion and character above all else. Encounter fun is important, too, but is separate from immersion, so the choice above is a false choice for my group. I really doubt my group is alone in this. Just as I really doubt you are alone in your views and preferences.
Except something like "what's in the next room" when the players have no clues to tell them is easy to determine. If Jezal knows about trolls and fire is not so easy to determine. Folklore would exist in his world. Perhaps he had a cousin who faced a troll once.
Because it involves backstory beyond the scope of the game events it is much harder to determine definitively.
So there is a difference.
Maybe, but unless that cousin was one of the rare true adventuring types in the world, he probably ran his ass off or hid. My players write out their backgrounds. Those backgrounds, while not exhaustive, give a feel and framework to what the PC's history was like. I use that to determine if knowledge is certain or uncertain. Part of that is also where the PC is from. A desert dweller would be far less likely to know about trolls than a farmer who lived near the Troll Moors. There would have to be something exceptional in the background of the desert dweller to have the knowledge, and something exceptional in the background of the farmer to fail to have it.
I'm not sure I understand what you mean here. Do you mean you've never accidentally acted on meta-knowledge?
I dive deep into my character and what he knows, doesn't know, or is uncertain becomes almost second nature to me. I'm just not going to screw up and act on say, trolls being vulnerable to fire when my character is certainly not going to know about it. It will take me less than a second to figure it out, and if the answer is uncertain, I will convey to the DM why I should get a roll. If it is certain, my PC will act or not act on my personal knowledge, depending on which way that certainty went.
Sure, I can understand that. I'd say the best way to counter this is to change the monsters up and keep your players guessing rather than having them pretend to not know things they know.
That's an option once in a while, but it is not satisfactory to do for many encounters. I refuse to play the game adversarially, and going out of my way to counter player knowledge is pitting me vs. the players. It's also not satisfactory to my players who enjoy meeting classic monsters. Why should I punish them for something that they aren't going to do anyway? My players don't cheat.
I'm speaking from the DM perspective on this. The DM is as familiar with the characters as the players. So creating encounters that hinge upon the disparity between player knowledge and character knowledge is probably a bad idea. Better to design the encounter differently. That's what I meant about avoiding that wrinkle. This is where the DM contributing to metagaming comes into it.
First, I never create an encounter that hinges on any disparity. I don't consider that when I make the encounter. If I'm going to have trolls, I really don't care if they end up knowing about the weakness or don't know. If the PC's know, great. If they don't, great. It only matters to me that the knowledge actually be possessed by at least one of the PC's and not be the result of metagaming.
Second, it's not "probably" a bad idea. It's a good or bad idea based on what kind of group you have. If you have a group that enjoys metagaming, but you don't, then it can be an issue. If you have a group that enjoys metagaming and you're okay with metagaming, or a group that doesn't like metagaming and doesn't care about that disparity, it's not a bad idea to have encounters with a disparity in knowledge.
I suppose Skill Checks could serve to help in the gray areas...but I would think that simply allowing PCs to know that trolls are vulnerable to fure is fine. It's no less arbitrary than deciding that they don't know. And more importantly, it keeps the game moving along.
And you can play that way. Just let the PCs know everything about monsters. My group doesn't enjoy that, though. Also, what I decide is never arbitrary. I always have a reason for allowance, denial, or uncertainty with a roll. It's never based on a whim.