It’ll come as no surprise that Peter Capaldi’s final regular episode as the Twelfth Doctor will make everyone cry – but apparently, the cast were left reeling not from the regeneration but from another scene apparently of particular importance to the storyline.
As part of her Access All Areas Doctor Who special, which airs on BBC Radio 2 later this month, Jo Whiley visited the set of Twice Upon A Time, only to find the cast and crew still in a bit of state from the day before. She explains:

“The day before there was a scene that they had to film, and everyone said EVERYONE was in tears. Because of the poignancy of the scene that they were filming, which had less to do with the Doctor himself and more to do with the storyline. And everyone was in pieces. “And when you watch the Christmas Day episode, you’ll understand. It’s really very very clever, and very laced with emotion.”

We naturally think the regeneration will be a tear-jerker, but this isn’t the first time we’ve heard earlier scenes are emotional too. Indeed, it’s no great surprise.
If we look to The Time of the Doctor, which saw the regeneration of Matt Smith’s Eleventh Doctor, the whole piece is about grief, about mourning, but also about carrying on. It’s an enormously sad episode (I’d say it’s the most upsetting Doctor Who serial, personally), and similarly written by Steven Moffat: the Doctor is forced to settle down; he has to leave his friend for the greater good; only get glimpses of the sunlight each day; then have the option to run, but find that he simply can’t leave Trenzalore. He can’t have that on his conscience. He even loses his mechanical pal, Handles, who stayed by his side for hundreds of years. And the Doctor concludes to his companion, “Everything ends, Clara. And sooner than you think.”
Yep, of course Twice Upon A Time will be a emotional!
Twice Upon A Time broadcasts on Christmas Day 2017 and Jo Whiley’s Access All Areas Doctor Who Special airs on Radio 2, 8-10pm, on Thursday 21st December.