Kathy Nightingale: What's good about sad? Sally Sparrow: It's happy for deep people.

The trouble with a series as it gets older is it can feel like a tradition, and tradition is the enemy of suspense, and it's the enemy of comedy. It's the enemy of everything, really. So you have to shake it up.

Witty and mean is easy - but fond and funny is hard.

I think of myself as a writer with a sense of humour rather than a comedy writer. Happy to tell a story with lots of jokes in it - I wouldn't know how to do jokes without the story.

Rose: 'If you are an alien, how come you sound like you're from the north?' Doctor: 'Lots of planets have a north!

So is this how it works Doctor? You never interfere with the affairs of other peoples or planets, unless there are children crying?

Amy: I had something I wanted to tell him. Stuff always gets in the way. Canton: Stuff does that.