Argh! :doh: I knew this one, but just waited too long. This is, of course, Death backstage - just before he tells the actor to FORGET. A few minutes from now, Death would be tap-dancing...
Incidentally, I love the BBC adaptations of Terry's books. They're always done with such care and detail.
Obviously it's impossible to cram every single gag, observation etc. in, but that's what the books are for...