Argyle King
Legend
No, I am of the opinion that a player cannot perfectly play the mentality of a character separate from the mentality of the player, especially when it comes to matters of intelligence and insight. It's easier for example for an intelligent person to adequately play an intelligent person with a different belief and ethical outlook than their own, than it is for them to play someone who is always clueless. And it's easier for a DM to do this with respect to an NPC, than it is for a player to do with a PC, because the player invests his ego in the PC and is generally emotionally committed to the PC's health and success. When it becomes a matter of victory or survival, it's easy to convince yourself to fudge and pull a Forest Gump trope - where the 'stupid' character you play has an intelligent person's insight but is by convention of the story unaware of his own insights and understanding.
All I can really do in response to this is to try giving an example.
As a player, I have a very good grasp of tactics when it comes to combat. That is due to having a military background in a combat arms MOS, and due to having used those skills in action. Likewise, one of my big interests is historical warfare and tactics. When it comes to that sort of thing, I would dare say I'm rather skilled.
In contrast, one of the characters I currently play has very little in the way of experience when it comes to combat. In fact, the character started the campaign as an escaped slave who had never even gone on an adventure. There have been times when his (the character's) decision making progress was very different from what mine would have been given the same situation. A big reason for that difference is because of my view that the character would not have some of the knowledge available to him which is available to me; as such, there were things I did not think about or consider when viewing the game world from his point of view.
I do recognize that I'm possibly an outlier when it comes to being able to do that. All I can say is that I've never had a problem separating myself from the mind of a character when I've really wanted to. It's not something I always do, but it's certainly something I can do. I've done it both as a player playing a PC, and as a GM playing the part of a NPC.
As far as your Forest Gump example, sure that happens. However, how easily that can happen might depend upon the system. There are systems which have mechanics to prevent a player from choosing actions opposed by the mentality of their character. In such a system, I might choose a disadvantage such as Honesty during character creation which makes it difficult for me to lie. In that same hypothetical game, there may come a time when I want to lie to the town guard about something my adventuring companions are doing; depending on the severity of the lie, I may be required to make some sort of roll to overcome my character's usual personality.
In general, let me address everyone that denies mental skills are of a different character than physical skills.
Suppose you wanted to run Tomb of Horrors for your group. And, upon floating this idea, you learned that one player had prior to your suggestion bought a copy of the module and memorized the text. However, the player says, "Don't worry. I'll just role-play my character and pretend that I don't actually know anything about the module."
Would you agree that the player could do this so perfectly that his play during the game was exactly the same as the play and experience the player would have had he not already known every detail of the module? That is to say, the successes and revelations the player (and therefore the character) has would be simply and exactly the same set of insights the player would have had they never encountered the text, and any failures would be exactly the same failures that the player would have had made in ignorance?
Because the only way you can argue that mental skills and physical skills are exactly the same, is if you can sustain the belief that metagame information never informs a player's choices. A character's intelligence is different than a character's strength because the character's intelligence - unlike his strength - is supposed to inform the choices that the character makes. But the character is in fact only an avatar of the player, and it is always the player that makes those choices. You can therefore dissociate completely a character's physical skills from the physical skills of the players, because these are pure action resolution, but you can never completely dissociate a character's mental skills from the mental skill of the player. A player's physical body does not extend into the game world, but the player's volition and insight does because ultimately the game world is a shared mental space.
I do not believe metagaming proves much of anything about mental character skills being different. It's just as possible for me to metagame my character's physical skills. For example, I know that in D&D 4E I can survive falls which would likely kill me (or at least cripple me) in real life. As such, the metagame informs me that I am willing to take physical risks that I otherwise wouldn't in a different system or in reality. As such, those are times when metagame information has changed how I used physical skills.
To answer your question, I do suspect many people would be tempted to use knowledge they have. However, it's very easy for a GM to work around that by simply changing parts of an adventure; even trivial changes can lead to a world of woe for someone who relies too heavily on metagame information.
Now, I would agree that it is difficult for a GM to enforce mental stats and skills on a player. It is difficult because such things are not strictly measured in the same way a physical score measures encumbrance thresholds and things of that nature. However, I do not believe it is impossible. One of the ways I believe it is very possible is by training your players to expect that relying too heavily on metagaming can lead to bad consequences. A second way is by doing what many game systems do; that is to reward roleplaying rather than encounter solving. There are systems which reward roleplaying with character points and character advancement rather than strictly equating success with XP.