The crucifix insists on the body, the wounds and the death where a plain cross shows only the sign of them, and the earliest surviving depiction of the scene it presents was made to mock a Christian.
The Alexamenos graffito
Scratched into a wall on the Palatine Hill around the year 200, the Alexamenos graffito shows a man worshipping a crucified figure with the head of a donkey, and reads, in Greek, that Alexamenos worships his god. It is a piece of ridicule made at the expense of a Christian, and it is the first image of the crucifixion that survives, which is a striking fact about an event that would become the central image of Western art.
How a church talks about suffering
The devotional crucifix divides sharply along a theological line. The Gothic crucifix presents a suffering man, sagging, bleeding, dead; the earlier Christus Rex presents a crowned Christ standing upright and alive on the cross, reigning from it. Which of the two hangs in a church tells a good deal about how that church speaks of suffering, whether as an agony to be entered into or as a victory already won. Catholic practice requires a crucifix to be visible on or near the altar at Mass, which fixes the body, and not merely the sign, at the centre of the rite.