É ator David Bradley
David Bradley appeared as the "original" Doctor in the closing moments of the episode, as Capaldi's Doctor struggled to resist his own impending regeneration. It confirms what some had already speculated: that this year's Christmas special will be a multi-Doctor adventure, with the story of this remarkable reunion continuing in December.
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Bradley of course steps in to the role originally played by William Hartnell – Bradley previously played Hartnell in BBC2 Who origin story An Adventure in Space and Time in 2013.