Moffat has revealed some details about how John Hurt’s mysterious Doctor came about. Speaking with SFX, Moffat said:
“Why not a mayfly Doctor, who exists for one show only? I’d often thought about that. Would it be weird in the run of the series to have the 45th Doctor turn up and be played by Johnny Depp or someone? Would that be a cool thing to do?
“There was also the idea that if you could bring one classic Doctor back, you’d actually, impossibly, want it to be William Hartnell. You wouldn’t want any of the others. You’d want him to come and say ‘What in the name of God have I turned into?’ That’s the confrontation that you most want to see, to celebrate 50 years.
“Going round and round in circles on it I just thought ‘What about a Doctor that he never talks about?’ And what if it is a Doctor who’s done something terrible, who’s much deadlier and more serious, who represents that thing that is the undertow in both David and Matt. You know there’s a terrible old man inside them. Well, here he is, facing the children he becomes, as it were.”