Throughout Doctor Who Series 10, much of fandom fell in love with Bill Potts, played wonderfully by Pearl Mackie. And it was really sad to see her leave in The Doctor Falls, especially as her conclusion felt too juxtaposed against the rest of the episode. Fortunately, she’s back this Christmas for Twice Upon A Time!
Although that wasn’t always the plan…
Writer, Steven Moffat, who leaves Doctor Who after this festive special, explains:

“[Bill’s involvement] came to me later on actually. I was just starting into the script and we had Mark [Gatiss’] character and we didn’t have Bill and I was thinking, ‘It’s just proof of format again.’ You need someone to whom to explain. Not to explain it to the Doctor; that’s not what the companion does at all. The companion makes it fun.”

And a very minor spoiler here, he further says:

“The speed with which, to give one thing away, Bill figures out that this daft old brush is the [First] Doctor is humiliating for the current Doctor. He’s horrified that she gets it so fast. Those scenes and just Bill’s perspective on it [were essential]. And I got so in love with and used to writing that voice in the show that I missed her, and I just wanted her to walk in and say, ‘Hey why is that TARDIS so much smaller?’”

Hopefully, this Christmas episode will prove a nice coda on Bill’s time in the TARDIS. At least the Doctor will find out she’s alive; last time he saw her, she was a Cyberman, sacrificing herself when he blew up Floor 507. Fingers crossed for some closure.
And it feels right that the Twelfth Doctor has a friend in the tale leading to his regeneration.
Are you pleased to hear Bill’s back? Or is the tragedy of The Doctor Falls lessened by the Doctor finding out his friend is now walking water?
Twice Upon A Time airs on Christmas Day 2017 at 5:30pm on BBC1.