I think it would be more reasonable to say that I require more of the DM. And, as the DM in this discussion, requiring more of myself is likely to get better results since I cannot control other people. I can control whether or not I alter monsters when playing with veterans. And if I don't, I hardly have any room to complain in my view when the players reduce the difficulty of a challenge by applying their hard-won knowledge to the situation.
Knowledge gained through out of character means isn't hard won in character knowledge. It also trivializes the in character stats that might allow a less knowledgeable player to have a more knowledgeable character. IA new player trying to play a loremaster who knew little more than what the DM provided (either automatically or as a result of rolls) might validly feel slighted when the rest of the party are playing characters who should have much less knowledge than his, but end up implementing a lot more because of the players' knowledge.
Also, for myself and every other GM I know, we put more (often way more) work into the experience than any of the players. Asking them to keep their player lore-knowledge separate from character-lore knowledge isn't really asking for much.
To you. To me, it's unacceptable to demand other people play a certain way when I could have fixed this issue myself with the stroke of a pen prior to it becoming a problem.
Sure, if your setting is mutable and lacks a set history. If it does, it is an imposition on the DM's world to change rakshasa's (etc) function just because a player can't be bothered to role-play in character--and that is what we are talking about. We aren't talking about a player having a valid difference of opinion from the DM on what his character would know. We are talking about a player knowing his character reasonably shouldn't know something and choosing to implement it anyway.
I'm going to bring up something that I think talk on ENWorld often fails to present the reality on. Maybe I'm misinterpreting what people are presenting, but here is what I see:
Disagreements about many of these sorts of things often come down to an assumption that players
want narrative world control. That acting within parameters that keep their area of control firmly on their character with that character's attributes is a chain binding them down that they are chafing against because it doesn't let them do cool things and play the way they
really want to play.
Hogwash.
Sure, I have no doubt that some players feel exactly that way. And I don't fault them for it. I don't even think it's necessarily about immaturity. It can also simply be a playstyle they are going for, a collaborative, narrative-focused style where they want a hand in shaping the world, and they
as a player want some control over what happens outside of control that their character would have. I run that sort of stuff outside of D&D, and I even have a defined mechanic (and expansion of the Inspiration system) that I use to allow that in D&D in a limited sense.
But that is by no means the only attitude for players to have. I know that often when I am a player, I actually feel
cheated if the DM is running the world that way! I want to
interact with a "living world", and in order to interact the world has to have an independent existence (in this case that is in the DM's head and in his notes/books/etc). If I am
creating a world I am not interacting with it--at least not in the same way. Do you get the same experience reading a story you wrote yourself as reading one that someone else wrote? Nope. It feels quite different. And a lot of time when I'm playing an RPG (and D&D in particular) I want the feeling of playing in something I had no hand in creating and have no ability to affect other than through the avatar with which I am interacting with it--my character. I'm not the only one who enjoys playing that way.
So while I'm not accusing any individuals in particular of this, I just want to make it clear that the idea that players are going to feel bad if they aren't given control over things outside their character isn't necessarily true. Some players feel bad if the DM doesn't hold his world consistent and unaffected by player narrative control or out of character interests.
I'm not sure some people have ever considered that possibility. If you've only ever played with narrative control-desiring players, then maybe it's a foreign idea, but it's a real thing. This isn't some sort of mean controlling no fun DM power trip thing.
I'm with iserith and Brad on this, but I have a further questions because this is one of those instances I frequently mention happening as a result of the DM being concerned about the wrong thing:
Does the character need to know that the creature before them is a rakshasha in order to choose to bless something?
Does the character need to know that the creature before them is a rakshasha in order to choose to use a crossbow?
Does any character need to be certain of the outcome of any action in order to choose to attempt it?
Because if the answer to all 3 of those questions are not "Yes, unquestionably" then you have (even if unintentionally) stepped into the territory of trying to police the thoughts of your player - Because if this is a guess or a whim for them, like it clearly is for the character because they have no background on the subject, the action would be acceptable, but if they know even though their character is making a guess or following a whim the action suddenly becomes unacceptable.
I see nothing wrong with a character making an appropriate guess. When I'm a player, I will often make an Intelligence check by myself just to decide it my character would know something I'm unsure of.
"Dang it, nothing seems to be working against this fiendish monster! Priest, try casting bless on the rogue's crossbow or something, while we try to grapple and tie up the thing." It would be a stretch, and we'd all know that out of character knowledge is involved, but once in a blue moon it would be worth a laugh.
I'm not sure I would consider it childish and unacceptable unless you had explicitly told them before that it is lore they would not know.
Is the rule at your table that they don't know any unique vulnerabilities of a monster they've never met. I've seen those tables and they make even less sense to me. If humans have been fighting trolls for thousands of years, wouldn't every human culture in a region where trolls are a threat know what is needed to defeat them?
I assume things like fire for trolls and silver for lycanthropes is pretty common knowledge in a typical D&D culture. I'm also negotiable on those things. If a player brings up something that they OOC think should be common IC knowledge, I usually end up agreeing with them (maybe it helps that my players generally provide solid justifications).
So if the answer is that the PCs will know the vulnerabilities of some of the monsters, then someplace you need to make it known that there are guidelines to which vulnerabilities your characters don't know. Unless this is made clear to start, it's hard to fault somebody for stepping over a line.
I wouldn't fault someone for incorrectly guessing where a line is. I would fault them for intentionally flaunting my generous interpretations by using something they themselves don't believe that their character would know. Fortunately, with my group it's probably more likely that I would tell them, "your character would know X" when they didn't think they would than the reverse.
Simply declaring player knowledge and character knowledge is not an acceptable parameter. It's too broad. Otherwise the players will spend an inordinate amount of the session trying to determine what their character knows or not.
Each player gets a brief document telling them what their character knows in general terms.
Here's an example paragraph from one character's primer:
"You are familiar with certain types of monstrous creatures that roam the world. You understand the nature of the undead. You also are aware of the existence of elemental creatures and magical constructs, as well as knowing that more bizarre creatures dwell out there."
From another character:
"You also have a basic understanding of various other races of the world, dwarves, orcs, etc, as well as monstrous species such as dragons or giants. You know the difference between good and evil dragons, and that their breath's vary, and you have heard of the five types of true giants. You understand the changing of the seasons, and can identify all common plants and beasts. In addition, you are familiar with basic truths about the fey. You know about those who have come back from the grave—the undead. A variety of types exist, and you have a better chance than most of knowing the difference..."
While it's nice to role-play the elf telling the human something new about a creature they have just spotted, we don't really have the time to play out the character's lives in real time. So much of that is glossed over as knowledge that is shared at a meal at a campfire, or over an ale at a tavern.
I like this thought. Sometimes my players like playing out in-character discussions about their past and the things they know, so assuming more of those have happened is a great idea.
The thing that I'm being critical of, and feel that it is ill-advised for DM's to encourage, is players actively using knowledge that they themselves do not believe that their characters would know.