If you told me that I had to make a film of Sherlock Holmes, and the only two actors I could cast were Jude Law and Robert Downey, Jr., there would be no question in my mind that Mr. Law would be Holmes and Mr. Downey would be Watson. Rachel McAdams would probably not be in this film; if pressed I could probably cast her as Irene Adler.
Sherlock Holmes is skinny, injects cocaine, plays a Stradivarius, and has a picture of the Queen on his wall. He also hates (and/or fears) women, excepting Mrs. Hudson. He hates everyone, mostly, but women especially and irrevocably. He can beat you up, but generally he can't be bothered or does so by trickery.
Watson is steady, sturdy, and dull. In addition, he adores Holmes. He does not sneer, because he is not sufficiently clever.
Neither character is a type. Both were invented and written by a single man named Arthur Conan Doyle. They have fixed personalities and tendencies, even if Watson's bullet wound repeatedly moves from his shoulder to his leg. Holmes is not King Arthur, Watson is not Cinderella. There is no room to maneuver.
If you want to make a film about Victorian bare-knuckled boxing and Jude Law in a bowler, you are allowed to do so. But you are not allowed to call it Sherlock Holmes.