I spoke with someone once who was In The Industry and she said it ain't just Caucasian, it's somebody with naturally thick, medium-long hair of medium to light color that is willing to let the $300-per-hour hairstylist do whatever the director wants to do to it.

Lighter colors are preferred because they show up better on camera than dark brown or black hair. (That's why most women on camera sport at least highlights if not a bleach job.)