Interestingly, Billie Piper, who returned for the episode, never had any interactions with her Doctor, the also-returning David Tennant, instead sharing all of her dialogue with John Hurt‘s War Doctor. And certainly she wasn’t playing the Rose Tyler we’d come to know. But Moffat explained his approach to Piper’s return in today’s roundtable discussion.
“I thought the story of Rose, which was beautiful, was done,” he said. “I didn’t want to add to it, and I didn’t feel comfortable adding to it, really. That was always [former Doctor Who showrunner] Russell [T Davies]‘s story. But we did want Billie. And I liked the idea of bringing back the Bad Wolf version. The way Russell ended it in “The End of Time,” by just sort of looping it around, was perfect. I didn’t want to stick another bit of it in. That would be wrong. I just wanted to get Billie Piper, one of the absolute heroes of Doctor Who, back in the show, but without interfering with the story of Rose Tyler. I just thought I might spoil something. I might trip over my bootlaces doing that.”
If Billie had declined the role, Steven says, “There would have been options” in terms of other Who actresses to bring back. But, he said, “none of them would have been as perfect as Billie, who, in some ways for me, represents the revival of Doctor Who more than anybody else. I know I’ll get in trouble for saying this, but if you look back at the first two years of Doctor Who‘s revival, it’s all about Billie. It’s all about Billie. It’s her show for two years. I mean, it’s really startling watching ‘The Runaway Bride’ and you’re going, ‘Where is she? Where’s the star of the show?’ So it was great to have her. We’d have done something else [if we couldn't get her.]”