MOFFAT: […] With Watson, the problem is when you remove the narrator function from him—because he’s really just the ideal audience for Sherlock Homes in the book. You actually have to do something more with him.
Our Doctor Watson is very sardonic and snarky and funny. But if you actually look at the original Doctor Watson, he isn’t; he’s endlessly credulous, constantly amazed—not quite Nigel Bruce—but [he] nonetheless has an epic ability to be wrong about everything.
[In Conan Doyle canon] he’s not as thick as he can sometimes be presented, but he is comically astonished by Sherlock Holmes’ deductions for the entire thirty years of their friendship. You think at a certain point he might know Sherlock Holmes has probably got this one — not saying, ‘You can’t possibly know that, Holmes!’ Not three decades in!
We’ve got an actor like Martin Freeman, and I think the thing that’s important for Doctor Watson is that he’s definitely hugely competent.
He’s not any kind of genius, but he’s a very competent military man, and a good doctor with a stout heart, and the best friend you could want, and the first man a genius would trust—which is a huge compliment.
A genius chooses him.
A genius who understands everything about everybody chooses John to be the man he trusts.
So that’s about as big a compliment as you can get, really."