Trivia: Possible Spoiler: At the time of this writing, it is generally believed that the "Mister Saxon" referred to throughout Season 3 will turn out to be The Master. One piece of evidence backing this theory is The Master's penchant for anagrammatic aliases: Mister Saxon is an anagram of "Master No Six," and if it's true, John Simm would be the 6th actor to play The Master. [Russell T Davies (who wrote most of the episodes) has said that the anagram is pure coincidence which was picked up by some people, but was completely unintended. Still interesting regardless.]
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Trivia: Very few actors have played separate villains in three different Doctor Who shows. Julian Bleach plays Davros in the main Doctor Who series, the Ghostmaker in Torchwood, and the Nightmare Man in The Sarah Jane Adventures. Paul Marc Davies plays the Futurekind leader in Doctor Who, something in Torchwood (I forgot) and the Trickster in Sarah Jane. Nicholas Briggs also provides voices in all three series for aliens and robots.