The Snowmen: At Your Service!

It's the shot where you can ­really see that the TARDIS is bigger on the inside than it is on the outside ... Because of the shape of the set and the studio, it was pretty tricky to pull off, but visual effects are a lot better than they used to be. The biggest complication was giving Matt Smith enough time to acquaint himself with where all the dials and levers were. Obsessive Doctor Who fans, like [writer] Steven Moffat, are very particular about the continuity of which bits turn round when the TARDIS is flying, and what lights react to what. Personally I'm not so bothered.

The snowmen were pretty difficult to get right. The ones we started off with looked like Zippy from Rainbow, and you can't have non-scary, almost cute-looking monsters. We were very amused when we saw the John Lewis snowman advert – I guess we've made their evil cousins!