Steven Moffat said of the casting for the character,
s t iː v ə n WORD_BOUNDARY m ɑ f æ t WORD_BOUNDARY s ɛ d WORD_BOUNDARY ʌ v ð ə WORD_BOUNDARY k æ s t ɪ ŋ WORD_BOUNDARY f ə ɹ ð ə WORD_BOUNDARY k æ ɹ ɪ k t ə ɹ WORD_BOUNDARY