On Friday afternoon I encountered someone my character (I?) really clicked with, and we spent all waking hours until bedtime Sunday night writing together. It was amazing.
I have no idea what causes that click. It's an immediate thing, I know when it happens from the first or second post. I don't know how much of it is the character or how much is the writer/real person, but I've come to suspect it's both. (I had RPed with this person previously, but I didn't like the character they were playing, and so nothing clicked. When he brought in this new character and we RPed, just *snap!* there was this connection.)
In my experience, clicking is pretty rare. It happened once in WoW, but that didn't last (a third party was ICly and OOCly jealous of the time the other person was spending with me, so did everything he could to break us up ICly and OOCly. Sadly the person I had clicked with decided to stop RPing with me to keep the third person from being OOCly unhappy... I guess that means one person can feel the click without the other feeling the same.).
I'd love to know what causes that sort of click to happen in RP or RL, but I don't think we'll ever know. It's like when you meet someone's eye and just know the two of you are on the same wavelength.
Whatever the reason it happened, it was a damned good weekend. :D