We often talk about acting as a collaboration. We know that actors work with writers, directors, and many other people to create a production that we watch. But rarely do we get to really understand what that means.

Watching The Doctor change as a new actor takes over is an opportunity to understand exactly what an actor brings to a part. The transition from the 9th to the 10th doctor may be the best example of this, because Christopher Eccleston left after only one season. The writers did not plan an ending to the Ninth Doctor’s story, and David Tennant mostly picked up the part exactly where Christopher Eccleston left off.

In his first full episode, the Tenth Doctor says repeatedly that he doesn’t know yet what kind of man he is. But when he is asked if he will be the champion of Earth, he replies, “I don’t know what kind of man I am, but I think you’ve just summed me up.”

Television is a medium in which writers can adapt their writing for the actor playing a part. When something plays well, the writers write more of it. When something doesn’t work, it is dropped. And so a character like The Doctor changes with each actor who plays him.

There are essential qualities that belong to The Doctor as a character. He is brave, he is clever. He protects the Earth. He enjoys having a human companion. He hates guns and values life in all its forms. He is loyal to his friends. And at the moment of this regeneration, he loves Rose Tyler.

But what is different from one Doctor to another? Each has his own costume. Each has his own catch-phrase. And each has his own temperament. These are the things that are unique to an actor. Because it is immensely enjoyable to hear Christopher Eccleston say, “fantastic,” but in another actor’s mouth, it’s just a word. And so the writers used that word as much as they could during Eccleston’s tenure, and then it became just another word. It took a while for the writers to come up with “allons-y” for the Tenth Doctor, but once they found it, they used it again and again, and it became his trade mark.

One of the things that’s fascinating to watch about Doctor Who is the evolution of the character as actor and writers work together to discover the best persona for The Doctor in his current body. And one of the great things about the concept of regeneration is that it allows us to feel the awkwardness of a new actor taking over a beloved role. As we feel awkward, so does The Doctor. And as the new actor settles into the part and the writers learn how to write for the new voice, we adjust, too, and hopefully fall in love all over again, just like Rose.

That was, of course, the result when David Tennant took the part. I haven’t yet seen any episodes with the Twelfth Doctor, but I look forward to seeing the old character become new again.