I'm not a real roleplayer.
I know people who, while playing in an RPG, will take on a role so thoroughly that their thoughts become, largely, those of their PC. In a LARP, they might sit in the corner giggling madly the entire game. In a tabletop game, only a fraction of their roleplaying occurs at the table. Much of it never leaves their head.
In general, I'm thankful that I am not one of these people. Oftentimes, they aren't great for the game. They usually care more about character integrity than playability and fun... and I sometimes consider them selfish roleplayers. They don't always share plot or make the in-game concessions to group play that are needed for a well-functioning game.
Of course, I'm not a perfect roleplayer by any means. I tend to bring maybe a bit too much of myself to the characters I play. One of the things that I really enjoy about rpgs is the ability to exercise creative problem solving. This means that my PCs should be reasonably good at it and inclined toward it, which limits the sorts of PCs I choose to play.
Similarly, I don't usually enjoy games where I play someone who lacks what I consider to be basic reasoning abilities. Some people like playing with a 'character filter' - an artificial construct that translates what the player hears into what the character hears. This might account for PC prejudices or mental illness. It was very popular among players of Malkavians back when I regularly LARPed. I can rarely enjoy playing a character who requires one of these - I usually run into to much personal cognitive dissonance when I am in a position where my PC would make a bad choice that I know is a bad choice and that I think my PC should know is a bad choice.
There are, of course, exceptions to this. Sometimes I enjoy playing someone paranoid... or a bit fanatic... but those are the exceptions rather than the rule.