I've read the rules. I know the math. I know, as the player, that X monster is vulnerable to Y damage and if I move through the door 4 squares into the room, my burst 2 will catch every baddie there. But in the 6 seconds since combat started (he's still surprised!), he failed his knowledge check and he doesn't know whats around the corner.
Suggestions for how to play with that knowledge gap? I've almost caused a TPK by not playing the most tactically beneficial way because I didn't believe my character would have figured it out yet.
This is a matter of taste, but in my gaming style, it is Totally OK to have more knowledge than your character; this is what we call "Being Good at the Game" and it's fun. Remember, there IS such thing as skill at role-playing games; people play in tournaments to test how good a player they are, after all.
Finch address this in his "Primer on Old School Gaming" by saying that the player's skill is the character's "guardian angel"; the player keeps his character alive even when the character doesn't know better. For instance if you recognize a Wight, don't say "Let's get out of here, he's got level drain!" but certainly say "I got a bad feeling about this one guys!"
It depends a lot on the preferences of the GM and the group you're playing with. Some people are really bothered by using out-of-character knowledge; others don't mind. There are lots of instances where it doesn't break suspension of disbelief because the character could reasonably have just made a lucky guess or been otherwise fortunate.
One compromise solution that I've heard is a general rule that players can use their knowledge in-game, but only if they can come up with an entertaining story that explains where their character learned it.
I think playing to the level of your character helps you get into the character more. It may sound self evident, but I think a lot of people miss out on that. If that means that instead of sneaking up on a much more powerful foe you call them out, then so be it. I believe you will find more entertainment in playing your character to their character than you will in playing yourself in a virtual world.
When I run a game, I love it when a player goes against their nature in order to allow their character to do what they would do, and I try to reward them for that, because I also know it's hard.
As a GM, when running a game that demands a certain level of tactical mastery to be successful, I expect players to use player knowledge and not character knowledge to make tactical decisions. It seems rather unfair to me, as well as unrealistic, to demand a strict adherence to character knowledge particularly when doing so could result in what amounts to character suicide.
In most game systems that don't revolve around tactical combat this issue isn't as pronounced, in my experience.
As a GM, you can control how effective player-vs-character knowledge. For low level characters or new monsters, I would use more vague descriptions as opposed to naming the monster. I would also often have some monsters be unique in some way.
Many of the answers here illustrate the great strides we've made in the play of RPGs. Very heartening.
In our quest for perfection, giving full consideration to the quandary of IC vs OOC knowledge, we can be overcome by principle and lose sight of the fact that we, real people, are the ones playing and having fun thereby. So if the style fits your group's mind-set, ease up a little; permit (if not encourage) just a leetle bit of 'unrealism' in the role portrayals. It can help avoid TPKs ;> and increase the fun.
But again, applause to all who conscientiously strive to maintain that IC/OOC division; a very mature, positive, and responsible attitude. Very kewl. :)
As a (role-playing) player, I look at it from my character's perspective. "Have I encountered this creature before?" If the knowledge is something that seems specific (tactical move, weakness, etc) I bring it up to the GM, usually asking if I should do a skill check to see if my character would be aware of it. Many times the GM goes along with it simply to keep the pace moving along, but it would really depend on group. If the knowledge seems like something that my character would have encountered at some point, or simply figured out because the character's Intelligence/Wisdom is high enough, then I just go with it.
As a GM, I like to push the players to actually think outside the box to solve situations. So requiring them to use knowledge that their characters might not know is no big deal.
It really comes down to how the GM/group prefers to handle the situation.
As a GM you can often combat player-knowledge by changing the rules of reality, e.g. a wight doesn't have level drain in this campaign but a Gelatinous cube does or something.
I had a player who once wanted to try to "invent" gunpowder in a world where it didn't already exist, and use it in a metal tube as a musket. I wasn't too happy as he decided to "experiment" with substances he knew in real life to produce primitive gunpowder. So in the end I simply changed the "formula" for gunpowder to one his player didn't know.
As a PC it's quite easy to behave as if you simply didn't know the answer. The only difficulty is if there was a chance you might have figured it out or guessed it if you didn't already know. It is unfair of a GM to pretty much ban that course of action. Perhaps an INT check is appropriate in that situation.
I prefer to refer to the character's own level when making meaningful decisions, such as what the character thinks of a particular moral question or whether they like an NPC. But when it comes to life or death questions, you can override your character's ignorance.
If your players are abusing player knowledge - instantly knowing an enemy's weaknesses - then try deceiving them with rumours and misinformation?
I don't think this is a problem for a player to solve, but one for the DM to. PCs should be expected to use all knowledge at their disposal, both OOC and IC, so long as it doesn't assault the plausibility of the situation. What exactly does that will vary from group to group.
If the DM wants to create a feeling of mystery and excitement at facing a totally new enemy with unexpected powers and abilities, he should create a new monster, not recycle an old one. This can be as simple as reskinning an old monster or as complicated as coming up with a totally new profile from the ground up.