Sets.
I have grown up in Philadelphia and until I found out differently I was convinced that they had shot scenes in City Hall and not a sound stage in California.
shareI wouldn't have known, either.
It's the sign of a quality show.
I'm from Seattle and you could easily tell that Frasier wasn't filmed there. The same with Gray's Anatomy. Totally unconvincing.
In contrast, there was a show called "John Doe" (also canceled too soon) and it really caught the feel of Seattle.
I don't know where John Doe was filmed, but as a former and sometimes Seattleite, it had me convinced.