I just read an interesting little passage; which, I believe, Anthony Shaffer used in researching The Wicker Man.
"The whole of the Gallic nation is much given to religious practices. For this reason those who are afflicted with serious illness and those who are involved in battles and danger either offer human sacrifice or vow that they will do so, and employ the druids to manage these sacrifices. For they believe that unless one human life is offered for another the power and presence of the immortal gods cannot be propitiated. They also hold state sacrifices of a similar kind. Some of them use huge images of the gods, and fill their limbs, which are woven from wicker, with living people. When these images are set on fire the people inside are engulfed in flames and killed. They believe that the gods are more pleased by such punishments when it is inflicted upon those who are caught engaged in theft or robbery or other crimes; but if there is a lack of people of this kind, they will even stoop to punishing the guiltless."
Julius Caesar -- The Gallic War, 6.17.