 The writers intended to play off the idea of a hidden character being able to hear conversations revealing secrets . An initial idea of Pemberton 's was to have a large parcel on a table at the birthday party , but with Maggie — on whose birthday the episode would be set — absent . Other characters would repeatedly ask where Maggie was , until it was revealed that her head was in the parcel . This idea developed into the possibility of a hollow cake hiding a character , a device used in the final episode . For the writers , there is humour in characters spending time talking earnestly to a cake ; they noted that the prop cake had features resembling a face . In the editing stage , the production team kept reintroducing the cake in order to remind viewers of its presence . The original script ended with a knife being plunged into a fake cake followed by the revelation , contrary to viewers ' expectations , that no one was under it at the time ; instead , the writers upped the drama in the story , making it possible that any of the characters injured . For the writers , the original plot was , in a sense , darker , as some characters got away with wrongdoing in a way they do not in the final version . 
