It was announced in June that Smith will leave the show in the episode after four years as The Doctor.
Moffat told Doctor Who Magazine: “Matt’s last one is tremendous. It’s a terrific episode, and Matt’s performance in it is mind-blowingly good.”
He added: “It’s the best one he’s ever done, and a contender for the best performance ever by anyone as the Doctor. Absolutely sensational. His final performance, and it’s absolutely knockout.”
The showrunner also discussed the arrival of Peter Capaldi as the Twelfth Doctor: “Of course, at the end of it, we’ve got a new Doctor. Peter has already been in to do his first scene… I’m writing Peter’s first episode right now, and the scripts for his first season are coming in.”
The Christmas special airs on Wednesday 25 December on BBC One.
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