"It's very happy-sad".
Also slated to appear in the episode is Pearl Mackie's Bill, who will also be exiting the series alongside current Doctor Peter Capaldi, and Mark Gatiss, a frequent writer on Who and Moffat collaborator (Sherlock). As revealed earlier, the episode also features Mark Gatiss.
The panel covered heartfelt goodbyes for the 12th Doctor, and both hope and praise for the incoming 13th Doctor, Jodie Whittaker. It then cleverly segues into Bradley recreating the scene and, excitingly, we get a glimpse of Doctor Numero Uno starting to regenerate. We won't spoil any of it for you, but really, you should have watched it by now.
As the cast beams about the Christmas event, we cannot forget that this will be the last time this brilliant cast will be together.
Moffat talks a bit about David Bradley's portrayal of William Hartnell's Doctor, saying, "He gives it such twinkle, such grace".More news: Melted fuel heap inside Fukushima reactor
While everyone is in a tizzy over the casting of Jodie Whittaker as the new star of "Doctor Who", let's not forget that we're not done with Peter Capaldi quiet yet. The British actress revealed to Hall H that she would be back for the Christmas Special but that would be her last appearance as the Timelord's companion, though "never say never" to her returning. In 2007 he played Professor Lazarus, and he also briefly popped up playing a different character in 2011. Capaldi also says the most iconic moment as the Twelfth Doctor has to be his introduction in the Doctor Who 50th: "The Day of the Doctor".
Peter Capaldi is all for the idea: "I think Jodie's gonna be unbelievable".
On Saturday, Capaldi told Empire he was both sad to be leaving the series and excited for its future.
TWICE UPON A TIME will debut on 25 December 2017, or 26 December 2017 if you're in Australia.