Sao Til

Within a game played by John Barrowman and Tim Ingham in costume, Ingham's Sao Til was an Aminopius criminal with a robotic right forearm, who once attempted to take over the Doctor's TARDIS.

Biography
Having somehow found his way into the Doctor's TARDIS, Sao Til was gloating about now having power over Time and Space over the console when Jack came in. Cocking his gun at Sao Til, Jack demanded to know who he was and what he was doing in the TARDIS. Realising that Jack was an ally of the Doctor's, Sao Til answered that he was inside his own TARDIS because he was, in fact, the Doctor in a new regenerated body.

Jack was initially taken in, and even tried flirting with this new Doctor, noting that his blue skin was rather attractive by Jack's tastes and that his hat was quite stylish. The charade quickly ended, however, when, asked by Jack about his robotic arm, the false Doctor answered that it was "his new weapon" — a neural probe capable of "paralysing [his] prey". Jack instantly disbelieved Sao Til and resumed threatening him, knowing the Doctor would never casually carry weapons, no matter what body they wore.

Sao Til revealed that he was an Aminopius from the planet Aminopia, part of a race of literal "arms dealers" — body snatchers specialising in the sale of limbs. Answering Jack's once more pointing the gun at him by threatening to paralyse him with the aforementioned neural probe and then stealing his brain, Sao Til resumed his ranting about world domination.

It was at this point in the role-played story that David Tennant, out of costume, interrupted John Barrowman and Tim Ingham's game, asking them to get out of "his" TARDIS (NOTVALID: Tonight's the Night)

Behind the scenes

 * According to the Sao Til website, his name, when combined with that of his species, Aminopius, was intended to create an anagram of animus spoliatio, Latin for soul robbing.