I've always found it a curious conceit that within a world of a movie, the actors in them do not exist.
For example, in the world of the movie "Jerry Maguire," there is no actor Tom Cruise, or at least, not with the grin and smiling face we know of as Tom Cruise. Of course there can't be, our Tom Cruise is busy being Jerry Maguire.
Or in the world of "The Bourne Ultimatum," no actor known as Matt Damon exists. Or if he does, no one notes his remarkable similarity to Jason Bourne.
From what I understand, Ocean's 12 did something weird through this by doing some sort of weird thing where they noted that Tess looks like Julia Roberts. Making this conceit that obvious just makes me cringe.