In my case, my interests are sort of two-fold, so anyone who works with either the Holocaust or the Romanian-speaking countries is a good fit for me in all likelihood. Even professors who work with the USSR have told me they could help me, as they're familiar enough with Moldova and Communist Romania to work with me on my area of interest. So it's kind of a fluid thing. "Fit" is way more than just the professor though, and is in some ways quite intangible, IMO. A lot of it has to do with how comfortable you'd feel in a place and how much you really want to be around the program's environment, the school, the town it's in, etc. I mean, you could find a professor with very similar interests to yours, but if he's in a program where you don't feel comfortable with the environment, you may not be as productive as you could be somewhere else.