What if I, at the start of the campaign as I assign my scores, I write down that I know trolls are hurt by fire? Is that good enough? I've stated in advance that my character knows it. Does that qualify for @
TheHobgoblin's stated criteria that I have decide in advance?
If one lives in a world that has trolls and trolls are encountered often enough that the average village has to deal with them at least once per a generation, it is very likely that the character has in fact heard of trolls and would have heard that fire is the best weapon to use against it. Something like that would be relatively common knowledge and lore among the denizens of the world and unless the character has an Intelligence and Wisdom of about 5 or so, that is something that should be character knowledge.
Really, I don't know why that is the example people want to bring up because it is a really bad example due to it being something people who live around where Trolls roam should be able to pick up. Unless they came from a world with no trolls and were suddenly dropped into the world, that is something they should know.
Similarly, using fire against monsters affiliated with ice and cold against monsters affiliated with fire should also be common sense. It isn't beyond the bounds of stuff a character should be able to figure out.
Now, if there is a rare monster who, for example, was effectively immortal except if an item associated with them is attacked and broken. That as long as that object exists, the monster can just keep coming back... Well, if there have been no clues that such an item should exist or could exist, it wouldn't be right for you to call it out as that sort of creature at first glance and then start seeking out that item immediately as the character should not know about this aspect of the creature.
But the character naming every ruler of every city in the world, knowing precisely where the secret cities are located, being able to correctly list all deities major and minor and identifying their symbols on a glance without any training in religion, being able to perfectly identify every spell when not a spellcaster and not having any training in arcana simply because you as a player read the campaign guide and memorized the spell list....
Correctly identifying the difference between devils and demons and explaining at great lengths the differences between them and the entire arrangement of the planes when your character has no training in any skill that would suggest this would be within their knowledge... Using the secret words that were written in the latest Elminister book as the words that will compel the goddess of magic to grant any wish you desire in order to get your wish spell, knowing precisely where Drizzt and Raislin are located at any given point of time based on their novel series and so expecting to be able to meet them...
Using knowledge of the module to identify every trap, every secret door, call out the motivation of every NPC and reveal the plot twists long before they even arise simply because you read the module...
The list could go on and on...
None of that is reasonable knowledge for your character to have or be able to utilize regardless of what you as a player have read or played or done. They are not things a character would remotely "figure out" on their own.
Unlike acting in the traditional sense, the portrayer (the actor) of said role (the character) is not being given a set of parameters established by a third party (the writer/director/whomever). The actor, in this case, is also the writer of said character. The sole determiner of the character's parameters. Therefore, everything established about the character is de-facto "in-character".
I see.
So when you play D&D with the people you play it with, you are used to being able to sit down and say "I am a force timelord whose current regeneration is that of a Gelfling and I have saved the universe so many times that I have been ascended to being a junior member of the Q continuum." and people just... go with it.
Well, that is fine for you at your table who are playing D&D in name only and whose character concepts are limited only by the bounds of their imagination... but that is not the game most players play.
Most people who play D&D have at least two major third party forces determining the parameters of the character. The first are the rulebooks themselves which place many limitations on the form your character can take with you having to make several choices during the creation of the character about what the character is strong in by giving up other possible options and simultaneously creating what the character is inept at. Race, Class, Background, Skills, Equipment... there are very rigid parameters placed on the creation of the character which I am sure are far more narrow than the bounds of your imagination are-- particularly if you have indulged a fair bit in fantasy and sci-fi.
Right there, most people who are playing the game have a major third party dictating the bounds of the character parameters.
The second major third party is the DM who builds the world and tells you about the various options that would be appropriate for the story they intend to tell. Now, the DM has the ability to loosen some of the restrictions that come from the rulebook. Maybe the DM wants you to start at level 5 or will allow you to play a Kobold Bard or a Ogre Necromancer or play a custom class or something that is otherwise taken directly from the main rulebook.
But, at the same time, the DM tells you what the world is like and who resides there and through this provides you with new parameters. Granted, plenty of DMs use published worlds which is tossing that responsibility very much back to the rulebooks, but nonetheless the DM's descriptions and instructions place many other parameters on what a character could imaginably be for the game and could have imaginably encountered prior to the start of the game.
Not very many DMs are willing to accept a character's background being... well, let's say a real life sci-fi/fantasy fan who played D&D to a massive extent before somehow being sucked into the game world... which is really one of the very few explanations one can use for a character being able to directly cite stuff directly from the monster manual and campaign guide and such.
So while those following the rules of the game as they are laid out, third parties have set out certain boundaries outside of which your character cannot be written, within those bounds you have freedom to choose from many different combinations of option-- but those third parties setting your parameters do in fact exist. This is setting aside yours where perhaps the force-using Timelord Gelfling Q is totally accepted on face value without question.
And one can say with almost certainty that those parameters preclude the character from having access to certain knowledge that you as a player no doubt have access to. If you are going to play the character, it is important to try to separate that knowledge and try hard not to abuse it for personal gain within the game.
Again, though, the whole using fire on trolls thing is a really bad example of using player knowledge in character because unless the DM is telling you that this is the first troll that anyone in the world has ever seen, then it's a pretty good bet that word about them has spread around a bit so that it isn't unreasonable for that to be character knowledge. Similarly, it is silly for DMs to have players roll Knowledge Nature to identify what a Kobold or Bugbear is because those are regular denizens of the world that are plenty common enough that the average villagers would likely talk about them and someone not being able to identify them is akin to someone in the real world seeing a bear or a mountain lion or a raccoon and being totally bewildered about what the creature could possibly be.
On the other hand, if an Aboleth, Couatl, Ixitxachitl, Quasit Umber Hulk or Xorn is encountered, there i far less reasonable that a character without the right Lore skill would be able to identify any of those things.