Here's a taste of what Stephen Moffat had to say about the mini episode:
BBC: How did The Night of the Doctor come about?
Steven Moffat: Well, we had our new ‘hidden mystery’ Doctor and I was thinking, what else can we do for our anniversary year… I thought… Why don’t we get Paul McGann in and regenerate him into John Hurt? I’d like to see that! I’d love to see that! And we had this possibility that we could do it as a surprise, so we got in touch with Paul who was dead keen and I’m delighted to say he was so happy to join in with the idea of keeping it secret. He was childishly excited about it!
Paul came along and shot it - it was the last two days of the shoot for the fiftieth, actually. He did a wonderful job. It was great! We designed a new costume for him based on his old one. Howard Burden did a fantastic job with that.
Read the rest of the interview here but make sure you watch the video first, deal?
“The Day of the Doctor” airs in the US on November 23 at 2:50 p.m. ET.