When it comes to actors who play Sherlock Holmes there is definitely a line of demarcation that we can exemplify with Robert Downey Junior's Sherlock versus Benedict Cumberbatch's Sherlock.
In the first category are well-known actors who take the role, and as we watch them portray Sherlock Holmes, we are rarely unaware that it is not that actor playing Sherlock Holmes. Roger Moore, Tom Baker, Matt Frewer . . . and, to an extent, Larry Hagman and George C. Scott, even though their characters weren't actually supposed to be Sherlock Holmes. Rare was the individual watching Sherlock Holmes in New York who didn't see the latest James Bond when they turned on their television, not Sherlock Holmes.
In the second group comes the actors who were unknown to us before taking up Sherlock Holmes, and convince us that this is what Sherlock Holmes looks like without the distraction of a familiar face. In this group we have the Rathbones, the Bretts, the Wontners . . . those who became either iconic or typecast, depending upon your point of view, as the face of Sherlock Holmes.
As the first popular actor to play Sherlock Holmes, and mainly on the stage, William Gillette's initial impact as Sherlock Holmes is lost to most of us. No one alive grew up with his face as their starter Sherlock. The video of his film adaptation wasn't available to most of us without a special effort only after we were already well Sherlockianly imprinted. But that element of being a stage Sherlock before film became the more universal entertainment does add a certain factor to our Downey-or-Cumberbatch equation.
To many, William Gillette would have been Robert Downey Jr. -- and already popular actor seen in previous productions. But to some, especially at that point in history, Gillette would have been their first living representation of Sherlock Holmes. A good share of those had to have never seen Gillette on stage before, since attending plays takes a certain amount of "right place at the right time," giving him the originality of a Cumberbatch. It would be a lovely thing to sent a time-traveling statistician to theater exits to check which was the greater share, but I think it's safe to say that both were probably significant numbers.
So he may have fit another category which I failed to mention at first: The category of Cushing, McKellen, and Ferrell (*Yoink!*) who all were known before but slipped so easily into the identity of Sherlock Holmes that we easily forgot their previous characters while they were on screen. As good as those fellows may be, however, coming into a new actor as your first Sherlock Holmes is a touchstone of the Sherlockian experience. Coming into a beloved first Sherlock in that first encounter is even more special, whoever that actor might be.
Because, in the end, it's Sherlock Holmes. And that's pretty great.