Although John Leeson has been the primary voice actor for K9 since his introduction, two other actors have voiced the character. David Brierley voiced K9 during Season 17 after Leeson chose not to return to the series (Leeson subsequently signed up again for Season 18), and Roy Skelton, better known for his work voicing Daleks, voiced the character in Destiny of the Daleks when all was required of K9 was a croaking sound due to the character contracting a form of robot laryngitis.
The original K9 model was designed by visual effects designer Tony Harding. One early concept was to have a small actor in a large Doberman-shaped costume; however, Graham Williams vetoed this, saying that the robot should not look like a person in a costume. The eventual design was closely based on Harding's third concept sketch. (DCOM: The Invisible Enemy)
He helped Starkey fight the Oroborus, (TV: Oroborus) free the Medes (TV: Alien Avatar) and help one of the last Aeolians find her mate. (TV: Aeolian) He saved a Centuripede and her young from being killed by Drake (TV: The Last Oak Tree) and absorbed the Hunger swarm which threatened to devour Earth. (TV: Black Hunger) He and Starkey went through the STM to 1963, where they rescued Jorjie and saved Darius' great-grandfather William Pike from arrest and Darius from erasure from history. (TV: The Cambridge Spy) He enlisted the help of the Ukkan librarian Yssaringintinka to rescue Starkey, trapped on the planet Urlic. (TV: Lost Library of Ukko) He helped Birdie, a CCPC whith implanted human DNA, escape from the Department. (TV: Mutant Copper) He discovered the last of the Etydions (TV: The Custodians) and taught Taphony how to be a friend. (TV: Taphony and the Time Loop)
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On 3 April 2010, the TV series K9 began broadcasting on the Australian Television Network Ten as part of a Saturday morning line-up of children's shows. John Leeson is the voice of K9. The first episode called "Regeneration" featured the earlier Mark I version of K9. The "older" K9 materializes in London in the future and in the midst of an incident where an alien species is threatening human lives. K9 sacrifices himself in order to save the humans, but one component survives that enables his self-regeneration into the new unit. The main human cast members include Robert Moloney as Professor Gryffen, Keegan Joyce as Starkey (K9's new "master"), and Daniel Webber as Darius. The first 14 episodes of K9 season 1 were broadcast mid-morning on Channel 5 in December 2010 with the remaining episodes scheduled for January 2011. Trailers for the series appeared on Channel 5 from early December voiced by John Leeson. Originally, it had been planned to split the series in two halves between the Christmas 2010 and Easter 2011 school holiday schedules.
Introduced into the plot in The Invasion of Time, K9 Mark II first actually appear on-screen in The Ribos Operation (1978). More mobile than his predecessor, Mark II exhibited the ability to sense and warn others of danger. He travels as a companion alongside the Doctor and Romana. Around the time of Romana's regeneration (the transition from actress Mary Tamm to Lalla Ward), the K9 character was explained as suffering from "laryngitis" to accommodate Leeson's departure from the series at the start of the 1979–80 season; for this time, he was portrayed by David Brierley, until Leeson's return for the 1980–81 season. When the Doctor and Romana travel to the parallel universe of E-Space, K9 is severely damaged, in Warriors' Gate (1981). The damage was such that K9 could only function in E-Space; when Romana decided to stay and forge her own path, the Doctor gave K9 to her. The character makes subsequent appearances in remakes of the unfinished serial Shada, alongside the Eighth Doctor (Paul McGann); the 2003 Shada audio play and webcast depict K9 Mark II as having returned to Gallifrey with Romana, now Lady President of the Time Lords, where the Doctor visits them. It is not clear if either of these audio appearances are considered canonical in relation to the continuity of the TV series.