Amongst the guest stars is a barely recognisable David Walliams as Gibbis; a member of an alien raced famed for it's cowardice. It has become somewhat of a tradition to cast popular comedians in Doctor Who, with previous parts for Simon Pegg, James Cordon, Peter Kay and Catherine Tate, and Walliams put in a good performance. He was given some of the best lines in an episode whose gag rate was a bit hit and miss.