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.

Though the man above might say hello, expect no love from the beast below

When writing comedy, you have to have the confidence to believe that there is only one type of relationship in the world, and we are all having it, that all men behave in the same way and so do all women.

I like that Helen Mirren has been saying the next doctor should be a woman. I would like to go on record and say that the Queen should be played by a man!

You take this cold, remarkable, difficult, dangerous, borderline psychopath man, and you wonder what might have happened to him had he not met his best friend, a friend that no one would have put him with, this solid, dependable, brave, big-hearted war hero. I think people fall in love, not with Sherlock Holmes or Dr. Watson, but with their friendship. I think it is the most famous friendship in fiction, without a doubt.

Every episode [in a TV series] is a challenge, and what's challenging in most episodes is the monster. You're always a heartbeat from the monster looking ridiculous. You really have to work so hard to make them not look like ridiculous when they turn up on the set.

Sarah Jane Smith was everybody's hero when I was younger, and as brave and funny and brilliant as people only ever are in stories. But many years later, when I met the real Sarah Jane - Lis Sladen herself - she was exactly as any child ever have wanted her to be. Kind and gentle and clever; and a ferociously talented actress, of course, but in that perfectly English unassuming way.