These two met and bonded over their mutual love for Doctor Who, and they worked together numerous times down through the years before finally working together on the show that had cemented their friendship (both on “The Idiot’s Lantern”, an episode Mark wrote, and on “The Lazarus Experiment”, an episode that Mark guest-starred in). Mark was one of the first people David told after getting the role of the Tenth Doctor.
I love fanboy friends.
Mark Gatiss from the Quatermass Documentary: At the end of rehearsal, we had mics in our hair and he (David) said, “We have to get de-mic’ed, I’ve got to tell you something.” I thought something had gone terribly wrong with the production… and he said, “Chris isn’t coming back to Doctor Who next year.” I said, “Oh God! Really?” He said, “Yeah, and they’ve asked me,” and it was like, “Oh my God!” As long as we’ve known each other we’ve talked about how we’d play the Doctor. It’s our dream come true, so it was extremely sweet. I think he was quite nervous about telling me somehow, as if I’d actually try to kill him and take his TARDIS off him.
Trivia: Mark wrote a part for his friend David in the episode “The Unquiet Dead”, but Russell T. Davies, who was working with David on Casanova and who already had David in mind for the Tenth Doctor, talked Mark into making the mortician character an older man so the part would no longer be appropriate for David — mainly because RTD didn’t want to use David in a small part if he was later going to be the Doctor.