It's transpired that the skull used by TV's David Tennant during his recent stint as Hamlet at the RSC was in fact a real skull, and was that of pianist Andre Tchaikowsky.
The musician died in 1982 at the age of 46 after suffering from cancer, and in his will, bequeathed his skull to the Royal Shakespeare Company in hopes that it would be used as a prop in Hamlet's famous Yorick scene. From 1982 until 1984, the skull was left to air on the roof of an RSC building, and then in 1989 it was used in rehearsals, but had thus far never been used for a performance. Tchaikowsky finally got his wish back in August this year, when his skull appeared on stage with Tennant for 22 performances.
According to reports, the reason it's taken so long for it to be used was that no actor felt comfortable using it. Director Greg Doran told The Telegraph: "It was sort of a little shock tactic. Though, of course, to some extent that wears off and it's just André, in his box". He added that he didn't want people to know about it in advance, saying: "I thought it would topple the play and it would be all about David acting with a real skull".