What an interesting question, my sister!
In my opinion, the answer depends entirely on the person you have gotten to know. For example, now that I have met Master Ubar Papa and woobie, I can "see" and "hear" them saying their unique catch-phrases. I know how Papa says "egawds, daughter" and how woobie says "If he dies, he dies!" Although Papa and his woobie don't look all buffed-out or bony-assed, that's just the window dressing. RAGNAR and woobie ARE their characters -- or, rather, their characters are them -- and knowing them in RT adds an awesome dimension to our role-play.
Conversely, it's hard to picture Vala as an icy, asskicking blonde after seeing her adorable nose crinkle in all its dainty brunette glory -- and I don't think Raziel's "bio-luminescent gaze" will ever again make me do anything but giggle. When you discover that a character's real self is quite different from the persona, it can totally blow the illusion. Then again, it can also make you appreciate the level of care that went into the creation of the character -- and just how well the chatter STAYS in character.
Then there's the third possibility -- at least in my mind. You suddenly "see" a character you never noticed before, simply BECAUSE you have gotten to know the person behind that character. ~soft smile~ Personally, I think that's the most rewarding and remarkable possibility.
As for role-playing with a complete stranger... That, I view in the same manner as meeting a new face on a busy street and striking up a conversation. A stranger must be examined, felt-out, deemed worthy of further interaction. But, to some degree, those first impressions are always an illusion, an exterior put forth by the other person. Those first impressions will ALWAYS be different from the knowledge you gain when you get to know someone a little better, get to know weaknesses and personal foibles as well as strengths.
I think the person you DON'T know is always easier to rp with in one sense, because you rely only on their words (no RT/VT crossing, no ooc stuff), harder in another sense because you don't know them well enough to second-guess how they will react (will they laugh at your joke or punish you for being a displeasing slave?). And then when you DO get to know that person, it depends on whether their "soul" matches their "character" that determines whether the "illusion" is strengthened or lost altogether.
