Well, thinking outside the box has to be up there.
Being driven by something. Daring to question, to seek answers that range outside convention, to follow the trail wherever it leads, to test with experimentation, and to put themselves at risk for the sake of uncovering true knowledge. And being a bit obsessive.
Also holding the loyalty of a valued assistant, which counts as no small thing when it comes to character.
I honestly don’t see much that the actors have in common. I’m sorry to say that Rathbone’s interpretation (and the scripts he was given) never really depicted the character of Holmes as he was in the stories (and for that matter, the classic movie treatment of Frankenstein is very different from the book’s, and that of his creation even more so), but I still love him in the role just the same.