Speaking to Sci-Fi Bulletin, Steven Moffat revealed some information about the upcoming Christmas special, The Husbands of River Song:
First of all, every year I have to contemplate that this could be the last one I ever write – because I hadn’t signed up for next year at that point. At that point I hadn’t – unless they fire me – which would be quite sensible! So, as I thought it might be my last one I thought I’d get River in, which would bring me full circle. Also, and much more importantly, I was very very tired and Brian was going at me ‘What are we going to do for Christmas?’ and it just cheered me up that River could be in it. I hadn’t written River for years and I love writing for her. Also Russell has been at me for a while ‘You can’t write her out yet – Capaldi and Kingston. It’s a sex storm!’ It’s not called Sex Storm by the way! I think he was in the middle of his Cucumber phase and I told him that’s not what happens in Doctor Who. ‘You’ve forgotten, you old fool!’ And they are very sexy together.