You want people that can actually understand the character that they are playing and accurately portray them. That's pretty hard to find, it is why fan fiction is generally so terrible. The writer doesn't actually understand the characters well enough to portray them correctly. It is another good reason to stay away, not understanding the characters. At least with original characters you don't have something to compare against. ^^ So they can't really say you're portraying them wrong, unless you were really specific on their personality.