A problem that's existed in roleplaying games nearly as long as there have been roleplaying games is firewalling player knowledge from character knowledge. Part of the fun challenge in playing the characters is dealing with the unknown and the surprises it deals. But what if the players already know all the answers, either from having encountered them in previous games or simply by studying the various rulebooks?
Suppose the GM narrates this dangerous creature wading in to combat:
A 9 foot tall creature comes crashing through the forest. It walks erect but slightly hunched over, its gangling arms swinging low to the ground. Rubbery green skin covers its muscular body. It shows no fear or hesitation as it lopes straight up to you, letting out a hoarse roar showing a mouth full of sharp teeth and raising its oversized hands with fingers ending in sharp claws to attack.
This is a troll. But do the characters know it's a troll? (Assume the word "Troll" is not painted on the grass at its feet. 😂) Moreover, do the characters know that trolls quickly regenerate damage from ordinary weapons and spells, and are only truly vulnerable to fire and acid? High level character likely would but lower level characters would not.
Sadly a lot of the time in gaming an experienced player will shout out, "It's a troll! Remember it regenerates and we need to kill it with fire or acid!" because they're familiar with trolls- from somewhere else.
This is a firewall violation. The players took experience they have and imparted it on their characters who were supposed to earn it the hard way.
It's disappointing when this happens. One player's thoughtless or selfish act sucks all the drama out of the scenario. Suddenly the characters know exactly what to do to overcome a challenge. They go about it with robotic precision instead of having to figure it by trial and error and their own wits.
On the other hand, when players have to figure out stuff like this in character it's glorious cooperative storytelling.
Years ago I ran a game with a small group where all of the players either didn't know what trolls can do or did a great job pretending. Their characters hacked the troll to bits and left it for dead at the edge of their camp... only for it to rise, at full health, a minute later and attack them again! Then they killed it a second time, and it came back for a third attack! By then the characters saw enough clues provided- the troll had been hit by fire or acid a few times, and those wounds conspicuously weren't healting- that they figured it out. The game was so dramatic that way, without some know-it-all blurting out the surprising plot twist right at the start.
The group I've been playing my City of the Dead game with has been pretty good about firewalling. When they have out-of-character knowledge about a situation they either keep it to themselves- and try not to let in influence their actions- or they ask me. "Hey, what does my character know about this monster/spell/object/etc.?"
I'm glad they ask, because I've already planned to tell them! They're of a level (6th) where it's fair that they know some stuff about the mysteries of the world. Plus, when I created these pre-gen characters I made sure to give them all one or more Knowledge skills. This was intentional on my part so I could allow them the ability to piece together and predict, to a fun extent, what's happening in the game.
When
they encountered a troll in Session 1, for example, I advised one of the players that his character knew exactly what it was. He had the right background to have learned about trolls from mentors and peers even if he hadn't seen one personally.
I passed him a note explaining what his character would know. He got to share that knowledge in-character with the rest of the group, increasing their appreciation for him as a uniquely valuable member of the team.
BTW
my note passing technique has had an unexpected benefit. Now instead of players competing to be seen as the smartest (i.e., most know-it-all) player at the table by blurting out un-firewalled knowledge, they count coup by who receives the most notes demonstrating that their character had important insights to share. 🤣