[Disillusionist]

I guess I'm not as ardent a believer in the dividing line between player personality and character traits that some players are.
In many cases, I think some of those players are deluded.

With the single exception of roles you simply -can't- play well, such as the aforementioned 'moron playing a genius', I tend to believe that most if not all of the character traits are either enhanced or subdued facets of the player's personality. Arguably, even if one creates a character that is a huge departure from the normal behavior set of the player, after extended play time, it just seems improbable to me that some of the traits wouldn't make it past the 'bleed barrier' in either direction.

Personality traits in players are far from some static behavior set, and as people age, they do change. Heck, part of that (theoretical) maturation process might well have its roots in roleplaying games, which are a barely disguised abstract of some of the formative roleplay experiences we engage in naturally as children.
Cops 'n' Robbers (although no one seems to be playing that any more), Cowboys 'n' Injuns (which has been placed on the politically incorrect endangered list), Tea Party, and my personal favorite, Post Office.

The best I can manage in my RP is to give license to some forbidden behaviors within the structured confines of an RPG in order to play characters that I -like- to think I could never become, or to subdue impulses of my own personality which run contrary to the character's background.

I suppose there truly are some RP purists who are somehow completely divorced from their character's mindset, and are exercising a skill that I lack when they RP. I can't imagine becoming -as- immersed in a character that I don't truly identify with in some fundamental way. I think often that players act out in character as they might in real life if some barriers were removed, but for social grace later claim it was 'all just RP'. That goes for bad behavior as well as truly tear-jerkingly good deeds.
For instance, I've played evil characters, but I don't believe that evil people ever truly know that they're evil (so much for Magneto's Brotherhood of Evil Mutants of yore), but rather, do things construed as evil by others for a purpose that may not be apparent. In that light, it's much easier to play a character that does horrific things, if their agenda is sufficiently watertight enough to permit some rationalizations that make such deeds seem innoccuous, or even necessary.

I'm still working on generating a truly humorless character just to test my theory. Thus far, he cracks me up more than my professional jester.

Go figure.

[Edited to add: My personal peeve: people with no literary nor lyrical skill at all playing bards. I know now why they weren't always buried.]