The Sinestro Corps, unlike the new ones coming in(with the exception of Agent Orange, obviously, because isn't he the only one?), has individual members with their own personalities. Fear is a pretty broad thing that allows for a lot of variation.
Whereas the Red are just mindless rage addicts, the Blue threaten to give me diabetes and tooth decay each time I read their dialogue, the Violet are just vessels of another macro-personality(the Zamaron queen), and the Black are, well, zombies. Only the yellow and the green seem to require minds of their own and encourage individual modification. The rest are nothing but fingers of a bigger hand. Is this by design, or did Johns run out of imagination after the first two corps?