“We are going to do the story properly of the Doctor having lost a friend and making a new one. We’re not taking that lightly. It’s not in one door out the other. It’s the story of how all that affects him, why he engages with somebody else and what’s going on with that – that’s all important.

What does Jenna bring to it? It’s surprising just how much the show changes with a new co-star. The Doctor is quite different with her, and the way you watch them is quite different. You watched the Eleventh Doctor and Amy arrive together. It’s like they grew up in the same sandpit, playing. They felt not quite like equals – the Doctor never feels like an equal to his companion – but you knew them equally well and they were equally important to each other. They formed around each other. And one of the interesting things about writing the Doctor is that he’s so responsive to the people around him. It’s almost like left on his own his personality would slowly disintegrate. He becomes what people want him to be, a little bit. So he’s Amy’s Raggedy Doctor.

With a different companion he becomes a slightly different man. He dresses differently. The mere fact that he’s so much taller than her suddenly reveals that Matt Smith is very tall, not, as people assume, about average height, because he was about the same height as Karen. He’s the senior man, not in the sense that he’s more important but he’s the one you know already, and he’s training up a new one, as it were. In these five episodes the Doctor is practically the adopted son of Amy and Rory. He’s gone from being the wonderful man from space – Space Gandalf, as he wants to be – to being that troublesome kid that they try and keep under control. They even talked about getting babysitters for him in one unfortunately cut scene. They love him, but they know he’s a big kid, they know they have to look out for him, check he eats and all that. Whereas with the new companion he’s back to being the mysterious spacefarer.

And this never goes away, this thrill – you want to see the reaction when you see it’s bigger on the inside, you want to see the count the hearts moment, you want the story to begin again. And that’s what it gives you. It gives you Doctor Who at its most iconic, because a new person is having to learn the rules – and you’ve seen that story how many times now? I don’t think you ever get tired of it.”

(source)