I have read a few fics which briefly or peripherally referred to this. Usually it's treated as though the character has a passing resemblance to the actor. This seems like a reasonable way to deal with it to me. I think it's kind of interesting to think about, from a pseudo-philosophical standpoint. In my opinion, because the character isn't the actor and visa versa, there is no reason whatsoever that they would exist in a universe without the other media their actors participate in. The character is an idea and thus exists totally independently of whomever plays them.

It's more interesting to me to think about fiction universes in which magic isn't hidden to the public, because they wouldn't have experienced any of the books and movies and tv shows with magic in them that we watch the same way, though they could have still existed. What would something like HP be viewed as in a society that thought of magic as mainstream?