a) an oxymoron, and
b) at its worst in movie theatres.
Cory told me that he believed the home theatre changed movie-going behaviour: people were simply behaving as if they were in their own homes rather than in movie theatres. It's an example of the inappropriateness of certain private behaviours brought into a public space.
The most over-the-top breach of manners we experienced was during Hannibal. We sat in front of a couple that insisted on giving voice to every stray thought that crossed their minds throughout the movie.
During the beautifully-shot scenes in Florence: "Damn, Italy is beautiful. We gotta go there sometime, baby."
Watching Hannibal Lecter overpower just about everybody: "Damn, he strong for an old man."
When Gary Oldman's disfigured character first appears in full view: "Damn, you ugly."
Cory threw them an angry glance and I turned around to shush them with each outburst of theirs. Each time we admonished them, they'd sheepishly make some kind of apologetic gesture and remain contritely quiet for a couple of minutes. Soon afterwards, something would happen onscreen, a new thought would coalesce in their brains and they'd vocalise once more.
During Ray Liotta's last scene -- a rather grim and gross one at that -- the guy behind us broke the stunned silence with his funniest outburst of the show:
"Daaa-yum! Hannibal be eatin' HIS BRAIN!"
Glarkware has a product that might help out in situations like the one I just described. For a mere US$3.50, you can purchase a pack of 25 business card-sized "movie manners courtesy cards", shown below:
According to Glarkware's site:
Handing one to a talker means that you don't have to make a "shush" noise even louder than the talking. The vague wording of the text gives the (false) impression that the cards have been distributed by the theatre chain, lending the card-giving an authority that your "shush" lacks.

(not registered, but my name's Alia)