Steven Moffat used the 16 years that 'Doctor Who' was off air to inspire the 50th anniversary special.
The show-runner was left angered when the BBC axed the sci-fi series in 1989, but he admits it allowed them to explore the Time Lord more for the special.
Moffat told BANG Showbiz at the 'Doctor Who' Official 50th Celebration at the ExCel Centre in London yesterday (11.22.13): "I do think there was an opportunity that opened up to us. There's a 16-year gap, What was he up to?
"The man that doesn't tell you his name, doesn't tell you anything about his family, where his children went, doesn't tell you why he was traveling with his granddaughter.
"What was he up to when we weren't looking at him for sixteen years. That's the full story."
Meanwhile, Moffat always had a hunch that Ninth Doctor Christopher Eccleston wouldn't agree to appear in the episode, but he was very "good" about it.
On the issue of Christopher Eccleston and his absence in 'The Day of the Doctor', the writer said: "I sort of knew that he wouldn't [appear in the special]. I know Chris a bit. I did have a couple of meetings and there was a moment, I suppose, a giddy moment when we thought, 'Would he actually do it?'. This wasn't a decision he took in a funk or because he was cross or anything like that.
"He was very measured, he was very kind, very gentlemanly about it. He's just a good bloke.
"If you look at Chris's career, this is not the sort of thing he does. The Ninth Doctor turns up for the battle, but not the party."