Steven Moffat thought the 2015 Christmas special would be the last episode of 'Doctor Who' he'd ever write.

Steven Moffat

Steven Moffat

The showrunner has revealed he decided to bring back River Song (Alex Kingston) for the upcoming episode because he was emotionally drained after saying goodbye to companion Clara Oswald (Jenna Coleman) and thought this might be his own farewell.

He explained: "We've just lost Clara, so I didn't want to go straight into a new companion. I'll be honest, I brought River Song back in because I thought there was a possibility I'd never write it ['Doctor Who'] again so that's be my goodbye. But also, I really fancied it. I hadn't written River for a couple of years and I'd missed her. I thought, given that I'm knackered at the end of a series, what would be a treat for me would be getting Alex Kingston back in to do 'Doctor Who'."

Moffat says the Christmas special will have a much lighter tone than the end of the current series.

He told Radio Times magazine: "It's towards the comedy thriller end of the 'Doctor Who' market. It's light, and quite emotional at the end. The big deal is Doctor No 12 encountering River and vice versa. It's strange because they're now, in our human terms, the perfect couple. They're both sexy older people ... Now, yeah, they could actually be married. It works.

The 54-year-old producer added that although it's a Christmas special, there's not much of a festive feel to it.

He added: "It's a great romp for Christmas day but there's not a lot of Christmas in it. It starts with a big bit of Christmas at the beginning and then we ignore it and go off on a chase, really, with Mr. and Mrs. Who. If you're a long-term fan of the show, you might be interested because there's been a lot of talk about the Doctor and River's glory years but we've never seen them. Here at last we see them as Mr. and Mrs. Who at the height of their powers. It's great fun."