If you turn your head to stare at something, you're a rubbernecker. The classic example of a rubbernecker is someone who turns his head to look at a car accident while driving slowly past it (as if his neck were made of rubber).
A rubbernecker might stop and gape at someone who's tripped and fallen, instead of offering to help. The very earliest rubberneckers were described as "someone who eavesdrops" or "someone who gazes around with undue curiosity." The term became especially popular after cars grew in popularity, first for sightseers in cars, and later for morbidly curious drivers.