Think about the last sequence in Monty Python's The Meaning of Life - I think John Cleese would make an ideal Discworld Death - this shows that he can do the obvious "menacing" bit that the character is based on, but he has the versatility to show the other aspects of the character - some of the bewilderment of Basil Fawlty that others around him are not seeing the world the way that he does would work very well for Death.