Steven Moffat has explained why there will be an increase of two-part stories in Series 9.

Speaking in Doctor Who Magazine​ #484, Moffat said: “We’re changing the rhythm of [the programme] quite a bit. For a long while, those 45-minute stories were the backbone of Doctor Who. They felt new and fresh and different. It just started to feel to me, that as a member of the audience, you were getting too acquainted with the rhythm of it. You sort of think, ‘Well, now it’s about time for the music to come up.’

“Writing the first two-parter that I had done in years (Dark Water and Death in Heaven) I just thought, I’m liking this. This feels more unpredictable.’ Because you don’t know how far you’re going to get through the story…”

“The rule I’ve got is that you won’t be absolutely certain whether a show is going to be a two-parter or not,” he continued. “And sometimes something that looks like it’s going to be a single, isn’t a single. I think the nice thing is not having the feeling of, ‘It’s five minutes until the end of Doctor Who, so he’s bound to start running now.’”

He added: “With each of the two-parters we’re doing, there’s a substantial difference between the two halves.”

Filming of Doctor Who Series 9 began in January, and will air in Autumn 2015 on BBC One​.