John Leeson played the voice of the Doctor's robot dog in 54 episodes of Doctor Who. His characterisation of the little robot dog was so skilled and so full of fun that he helped make what was due to be a temporary character into a fully fledged member of the TARDIS crew, much loved and remembered by fans. It is a tribute to the actor that his cry of Affirmative is as much associated with Doctor Who as the Daleks' cry of Exterminate. He endowed the character with a life force that saw K-9 return to the series and its spinoffs many times over the years.

John Leeson was born John Ducker in 1943. In the 1950s, he joined the Leicester Dramatic Society before winning a place at the Royal Academy of Dramatic Art in London. At RADA, he adopted the stage name John Leeson, derived from the surname of a wealthy godmother whom he believed was funding his tuition.