I admit a Ferrell or Stiller Doctor would be a bit much. But you could find a good guest role for them. Doctor Who has worked its way through all of Equity since its returnedand another sign of Whos flexibility is that there are roles for garlanded acting knights and pop stars (name another show that guest-starred Sir Derek Jacobi and Kylie Minogue in the same season?). And to Skaro with typecastingwho would think David Walliams would be such a revelation playing space mole Gibbis in 2011s The God Complex? And as pretty much every actor in Britain has appeared by now (off the top of my head only Helen Mirren and David Jason are needed to score Character Actor Bingo), it might be time to raid another country for the next Sharaz Jek, Davros or Miss Hartigan.