...the thing that's most fascinating of Pompeii is these plaster casts of the dead, which you see all over the site and in the museums, which somehow have captured the dead at the moment of their death. Not just humans, there's a lovely dog who obviously died just as he was trying to chew his lead and get free.Nowadays, plaster casts are no longer made because plaster casts of body cavities destroy the bones in the cavities.
They're a rather clever invention really of the late 19th century because the main excavator of the site at that time realised that when they found a body, what they found around it was a hole in the lava, and he guessed that that was the hole that the decaying flesh and cloth and clothing had left after the lava had covered them, therefore leaving a body-shaped cavity. He had the bright idea of shoving plaster of Paris into it, and this worked brilliantly because what it calls up when you get the plaster of Paris set is that image of the disappeared body with all its clothing in its contorted positions. You sometimes almost see the terrified expression on the face. So they're not real bodies, they're a cast of the hole that was left when the bodies decomposed. It's a rather ghoulish thought, but it makes terribly, terribly evocative images of these real people from the ancient world. (source)
These casts of voids (best way I can describe them) have been on display before, and even featured in early film travelogues of Pompeii. Last winter's BBC4 series, The Thirties in Colour featured an excerpt from the 1938 short The Eternal Fire, where one of the individuals involved confessed to having dragged out the plaster casts from the museum and arranged them artfully in the streets of the ruined city for effect. (They also confessed to having made a ruined fountain flow again with the help of a hose from a bicycle pump.)
You can read more about the history of the plaster casts in a book by Eugene Dwyer.