David was in rehearsals for The Quatermass Experiment when he was told that he had the role of the tenth Doctor. The 2005 BBC Four production went out live to the nation on April 2, 2005, fourteen days before the official BBC press release confirming that fact. Jason Flemyng, who played Prof. Quatermass, was supposed to say "Good to have you back Gordan" to David Tennant's character, Dr. Gordan Briscoe. However, during transmission he delivered the line as "Good to have you back Doctor" as a nod to his upcoming appearance in Doctor Who.
That makes Jason the first person to call David Doctor on air!