I thought the story of Rose, which was beautiful, was done. 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.