In the Christmas issue of Radio Times, Steven Moffat describes how he wrote the Christmas special in a hotel room in LA during the volcanic ash cloud earlier this year:

"I’m trying to think Christmas!  Crunchy snow and roaring fires! Scarves and stamping your feet warm and spicy wine and mince pies. Clearly I was in the wrongest place ever! Two days of writing nothing, and I took emergency measures. I downloaded every Christmas song I could find, closed all the curtains, and turned up the air conditioning to Doctor Zhivago, and sat at my desk in a big coat and mittens.”

The Radio Times Christmas edition is on sale now until December 27th. You can see a gallery of Doctor Who Radio Times covers on their site here.