I was in the British Museum a couple of years ago when I came across the death mask of Oliver Cromwell in one of their display cases. Oh, I thought, so that's what he looked like. That's the bloke who killed a king and helped drag Britain kicking and screaming into the 17th century. Oh, and killed a bunch of people, many of them Irish.
(And even a couple of kilos of plaster of paris can still be controversial).
You don't really get a sense of a person from a painting, at least I don't, but with a death mask you can look at the shape and size of someone's head, and imagine them lying there, asleep, about to wake up and invent gravity (as in the case of Newton).
They were created as soon after death as possible, to get the best likeness. The one above is of Beethoven, and obviously was once dropped by someone. oops. Other famous death masks include Jonathan Swift, Dante, and Hitchcock, oddly enough. For anyone who's as morbid as me, there's an excellent list here with pictures of Princeton's collection.
In the immortal works of Nick Ross, don't have nightmares.