So how do you balance these two sides of the equation? How can you keep them separate? I have some tips.
If your character is a blank slate who knows nothing of the larger world, then everything will be new to him. Look to the boys Rand Al'Thor, Perrin and Matrim from the Wheel of Time series. The three were plucked from Two Rivers with no experience of the larger world. This technique means everything is a discovery and no hidden knowledge could seep into the characters' brains. There's no danger of in-character to out-of-character bleed when all the IC stuff is brand new.
Of course, the close cousin to knowing nothing is "know only one thing." This is a style of background in which your character is inspired by a single ovewhelming event. Perhaps your human took up adventuring after Deathwing swung by and burninated your thatched roof cottage. Your average character won't know why the earth has just sundered and the Thousand Needles now makes a convincing waterpark -- but that guy who had his house dragon-breathed down? He has a fairly solid clue.
Then focus on that one bit of knowledge. Do you know anything about the Dark Portal? No, why would you? The Dark Portal didn't take a detour by your house and burn it down. Do you know anything about Arthas and Yogg-Saron? Of course not; neither went matchstick on your village. Anything that isn't related to your character's "thing he knows" is unknown.
You can start a new character with the best intentions to keep its in-game knowledge absolutely pure. But the stark reality is that as stories progress and your character learns the dark secrets of the world, it could become difficult to keep track of "what do I know" versus "what does my character know."
I advocate the use of notecards for these purposes. Jot down the knowledge your character learns, and maybe even the time and date. This will help you keep a better idea of the special bits of lore in your head and has the side benefit of making your case more defensible if anyone calls you out for metagaming.
Just don't play with that stuff
Another good way to keep your character's knowledge under control is not play with the secret knowledge in the first place. Sure, maybe Garrosh and Thrall pinky-swore to be best friends forever, and maybe they have each other's names tattooed on their biceps ... but why is Generic Joe Troll ever going to talk about it?
Do you think our generic troll is busy gossiping about Thrall's social life? I would expect he'd be confused by the recent actions taken by his beloved Warchief. But mostly, he'd go on about his business doing the things that trolls do with their day.
Avoiding these troubled areas of mixed IC and OOC knowledge will help keep your roleplay pure. More importantly, this will keep the roleplay focused on your story instead of the ones being written by Blizzard. That kind of tight connection with your character and your story is the point of roleplay, so you're getting two benefits for the price of one.
Keep it clean
The last tip might seem like the most obvious way of keeping your character's knowledge clean, but it's also the most important. Just be aware of the things you're talking about. Be aware, on a meta level, of what your character is saying. Only you can control metagaming, and only you are really aware if your character's seemingly outrageous knowledge is justified.
Just the simple act of being aware of this issue will help keep it in hand. You don't always have to play the character who knows everything; it can be just as rewarding to play a toon who is discovering the wonders of the world. If you let yourself give it a try, you might be surprised.
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