What do you think about the situation of encountering a rare monster that you've probably read about in a monster manual so you know its weakness while you know your character doesn't. How do you play that out?
It is difficult to not allow player knowledge about monsters or encounters of any kind to influence the actions of the character. For example, as a player (And being a Game Master) I know the weakness of certain monsters, like trolls being susceptible to fire in some systems, but my character has never encountered a troll. It's hard not to guide my character toward that realization especially if I am particularly fond of my character and don't want to see him eaten by trolls.
Sometimes this results in "I light a fire arrow and shoot it at the troll!" where did that idea come from? For no reason whatsoever the character just decided "Lets try fire." Often I see players trying to do this through rolls like insight or maybe some form of knowledge roll
"Does my character know anything about this creature?" *rolls knowledge/insight*
"Yeah, actually your character remembers hearing a tale once about trolls and how they hate fire"
This is better, in my opinion, than just straight inserting player knowledge, but it still seems a little off to me sometimes and a bit clumsy if you will. Especially when the characters are encountering a monster that has never been encountered by their culture how are you going to justify giving the player this information when there is no reasonable means by which they should know it?
Another way I might handle this, as a Game Master, is to alter the monster from its original design or as a player consider that the Game Master may have done so. Trolls are not susceptible to fire, for example, in fact fire heals them. Maybe I make trolls susceptible to water or a certain type of herb.
Edited: Aericsteele on 28th Sep, 2016 - 6:34pm
I also play it by what my character would know and not what I know myself. That makes it more challenging and realistic. I go for as much immersion as I can in character. Using my own knowledge breaks that. Almost all groups I have played with really frowned upon using player knowledge. Role playing is about telling a story. Think about a fantasy novel you might have read. If the protagonists had prior outside knowledge (From outside the story) of what they were facing it would be unrealistic and would ruin the story. It's the same way to me with role playing. No offense is meant by this. It's just my opinion and the way I play.
I agree with Aericsteele's method of allowing knowledge rolls and in Pathfinder the rules allow for different difficulty depending on the rarity of the monster. I also had a Dungeon Master that made the roll easier depending on the time you take to study it.
Another resource often forgotten are the town locals. If the monster has been pestering them recently or it's steeped in legend, they would know stories of its abilities. There can even be some misinformation regarding its weaknesses that come as a shock to players when they don't work (Vampire myths come to mind).
Personally I find it a challenge to separate my knowledge from my character, so I try hard to keep my character's decisions out of my hands. If I know a monster has a particular elemental weakness or resistance that my character doesn't know, I would choose a random energy type to attack it with and experiment until my character figures out what works.