What I'm looking for in an RP partner is someone who can post at least two or three good paragraphs per post. I don't want something like:
"Snape walked down the hall skulkily, hatred for James boiling inside him. He stepped into an empty classroom and slammed the door. While he looked around the darkened room he thought to himself.
He wondered why Potter and his lackeys were so downright mean. His fists clenched. Then he heard a knocking on the door."
That technically is two paragraphs, but IMO it's not a full post. Obviously that's just something random I threw together for an example, but you get the idea.

Anyway, someone who posts quite a bit more than that.
Another thing is, I'd rather it be someone who's read the books, because there's more information in them than in the movies, and I might intentionally or not bring up details from them that were left out and confuse someone who's only seen the films. I won't turn away someone simply for only having watched the films though, just saying it would be smoother if you have read them!
So if anyone's interested, please let me know.