I think there are a couple of things at work here. First and most obviously, our group has a comfortable sort of camaraderie that makes it easy for us to put it all out there. But there's more to it than that. Thanks to a miraculous alignment of stars, we all have characters with strong and interesting personalities and with equally interesting flaws and quirks. That doesn't always happen. In fact, it seems to be a bit rare. In Knights of the Astral Sea (my other campaign that I'm running right now), we don't seem to have that same character magic*, even though we've had some brilliant sessions. And looking back at games that I've played, I can count on one hand the number of times I've had a character who was as easy to play as Erik Sorensen (my Slaying Solomon character when I'm not directing).
Another thing that fascinates me about these stand-out characters is the ease with which I can summon social skills or personality traits that are far removed from my usual self. I'm specifically thinking of one particular character who was more take-charge, take-no-prisoners, make-them-flee-in-terror thna I could have hope to be. That I'd ever want to be. Where does that come from?
And one more weird thing... these special characters that are so easy to role play are not limited to the ones I bring to the table for long-term play. I've had a few one-shot convention games where I achieved that degree of character immersion that is so often lacking in longer term games. Why is that? Is it because a particular social dynamic at work? Feeding off the energy of a great group of people that you don't normally play with? Or is it because one-shot characters are often caricatures with exaggerated personality traits? Perhaps that is a useful lesson to remember the next time I create a character for a game.
* Well, except for possibly Genevieve