The other, related, question is: what if the player knows, but the character doesn't?
No. Unless their background or skills mean they should know.
In my opinion the creators of the Walking Dead had it right. If zombie movies exist then zombies are less scary. We played Curse of Strahd the same way. No characters knew what this blood sucking fiend was, because otherwise their characters are all planning for it.
If the players are mature enough to separate character knowledge from player knowledge I find this method is far more satisfying.