I have mixed feelings about this myself. They're always going to realize they're playing a game, that's true, but I would like it if players made an effort to separate their knowledge from what their character knows. As a player, I might have strong suspicions the really pale dude named Wormtongue can't be trusted, but my character might not have any such notion. I do agree with you though, going too far to prevent that kind of thing is only going to call more attention to it. As a GM, when players are using out of character reasoning for the actions of their characters I generally just roll with it.Here's the thing.... the players, even if they're trying to think entirely as their character when they make decisions... will remain aware that they're playing a game and that the game elements including all the NPCs are created by the GM. So very often they will or won't trust a given NPC based on nothing other than player intuition. And I personally don't think there's anything wrong with that... it's unavoidable and trying to prevent it only calls more and more attention to it.