Polanski has a pronounced fondness for, and excels in, what in French is known as a 'huis clos', i.e. a drama set in restricted space, filled up by ACTING. 'Carnage' is all drama performance, the staging seemingly invisible (which obviously is not to say its not there; quite the contrary).
Limiting the film to atypical 1h15 was an astute decision; the decidedly Bergmanian quality of this face to face with its subsequent fall of masks actually makes it fairly tiresome!
PS, though! A story that begins with a departure! What a stroke of genius!