Lately I've been having the same experience over and over--the surprising disorientation of reading terrific work by people whom I previously knew only socially. And each time I'm surprised to be surprised: these are friends! People I know are smart! People with whom I've discussed projects and presses and publication strategies. And yet the experience of reading their work is both strangely estranging and a bit like falling in love.
It is, I imagine, similar to the experience a child has upon realizing that her mom or dad isn't just her mom or dad, but is also a trial lawyer or VP for marketing, and thus has another life totally unlike the one the child sees and knows.
Maybe this response is unique to me--the result of not having had friends in grad school with whom I exchanged work, or of being in a niche sub-field. But I suspect it isn't, or isn't totally: our academic friends, the people we hang out with at the conference hotel bar or have over for dinner, are friends. We like them because we like them. Knowing what a person's vita looks like and complaining about work and swapping professional advice isn't the same thing as reading her prose. Even hearing a person deliver a conference paper isn't the same as reading the eventual article: the ideas may be the same, but even the strongest conference paper doesn't fully convey the writer's voice or the way her mind expresses itself in the silence of the printed page.
It would be going too far to say that reading a friend's work is like encountering a totally different person, but it's a bit like encountering your awesome friend's awesome sister: there's clearly a continuity between them, and as time goes on it will become impossible to think of the one without the other--but at first blush all you can see are the differences: this one has pink hair and used to be a professional archer; that one works for the State Department and collects netsuke.
(The opposite experience--finally meeting the real, live person who wrote a book or essay you admire--can also be disorienting, but it's somehow more expected. We know, at least intellectually, that an author is something other than the human being who walks around bearing that name, just as we know that the person we see on the screen would not be the person we'd meet in the street.)
As for discovering that someone I thought was awesome in person is actually a crappy or pedestrian writer. . . frankly, I've never had that experience. There are certainly people in the world who are smart and hilarious in person but who write abysmally flat prose--a phenomenon I truly didn't understand when I first encountered it in high school--but literary studies probably selects for those who are better than average at matching writing voice to personality; I know I select for it among my friends.
But if this is a surprise, it's unquestionably a pleasurable one. So friends-whose-work-I've-just-read: it's not that I didn't expect your work to be smart! I just didn't expect it to be smart like this.