Peter Capaldi made a brief cameo in Saturday's landmark episode, ahead of his official debut at Christmas.
"It was the plan from the start - all the Doctors… all the Doctors will fly in to save Gallifrey and change the timeline," Moffat told the press at the official Doctor Who Celebration at London's ExCel.
"That meant we should have the new one. I knew there was going to be a new one, so I wrote it not knowing who would be doing that [scene]."
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The BBC sci-fi drama's showrunner described Capaldi's brief appearance as "another treat" for fans.
"We did manage to cast somebody where you can use less than half his face, for less than a second and everybody knows who it is!" he remarked.
"It's just another treat - I was just trying to throw treats at the screen!"
Doctor Who will return to BBC One this Christmas.
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Watch a teaser for Doctor Who's Christmas special below: