Sao Til

Sao Til is an alien being once met by Jack Harkness in the TARDIS. Tim Ingham

It was later revealed that Sao Til was actually Tim Ingham roleplaying an alter-ego, he wore a blue mask, a black bowler hat and a huge robotic arm. John Barrowman was also roleplaying his famous character Jack Harkness in the TARDIS set. They had a faux-battle, before they were interrupted by David Tennant, who asked 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.