All of this is right on, especially about playing a character that really grabs you and you find interesting, because that enthusiasm and investment (with the normal caveat of avoiding getting ridiculously invested, here) leads to great rp all around.

I'll also second that, even when playing a very obscure character from a very obscure canon, I've ended up with canonmates solely because people I played off of were interested in my character and, by extension, their canon. I actually can't remember playing a character in a game that didn't end up with at least one castmate eventually.