One of the problems with Mozart's operas is that we're so familiar with the style of his music, with the sound of it; it's so elegant and beautiful on the surface and we tend to forget that the music in his operas is there to express exactly what's happening in the drama. If a character is melancholy, or joyful, or vengeful, or confused…Mozart is able, with an incredible amount of specificity, to communicate all of those feelings or characteristics in the music in a highly stylized way. Often he is successful in even giving us the sub-text of a character's words or actions, of expressing simultaneously both the superficial meaning of a character's words and the character's true feelings underneath. The greatness of Mozart's operatic music lies in this ability to musically portray human beings through beautiful, but ultimately human music.
The conjoining of Mozart's music and Lorenzo Da Ponte's intuitive and sensitive poetry provides a theatrical experience of the highest order. There are literally hundreds of moments throughout the score of Così where we hear the composer commenting on the text or the drama. One obvious example is the use of lengthy trills in the woodwinds to accompany the 'doctor's' use of the mesmeric magnet in order to restore the poisoned 'Albanians' back to life in the Finale to Act I. But there are more subtle points of comedy, such as in the Quintet, "Di scrivermi ogni giorno". Having been informed that the boys are going off to war, the girls admonish them in long, elegant phrases, to write every day; and in equally poignant vocal lines, the boys agree. On the surface, this is a perfectly gorgeous example of an 18th century 'addio' ensemble. But remember: this is a quintet. The fifth singer is, of course, Alfonso who, underneath the beautiful surface, is laughing at the overwrought emotions of the lovers, right down to a quiet little 'chuckle' of eighth notes in the last couple of bars of the piece. This is Mozart at his best. Using very economical means he has three levels of emotional communication occurring simultaneously: the girls, seriously thinking that the boys are going to war, are bidding them farewell, possibly for the last time. The boys, knowing this is all a ruse, are bidding the girls farewell but with tongues placed firmly in their cheeks. Alfonso, who put the entire farce in motion with his jaded view of women, is commenting on the action from the sidelines with his cynical wit intact.
As for musical characterization, a couple of examples will suffice. Note the use of regular phrases and a pastoral, or folk-like, quality to the arias of Despina. This know-it-all housemaid is not of the same social class as any of the other characters in the comedy, and so Mozart chooses to treat her music as if it comes from another world, the world of peasants, shepherds, milkmaids and farmers. They are an earthier bunch than the Fiordiligis and Guglielmos of the world, and we find that Despina is much more in touch with the vagaries of romantic (and probably physical) love. On the other hand we have the higher-toned, noble sisters whose musical world is borrowed directly from the contemporary vogue for Italian opera seria in Vienna at the time. Listen to Dorabella's adolescent outburst "Smanie implacabili" with its orchestral flourishes and angular vocal line. Mozart deals with her sister Fiordiligi in a similar way in the aria "Come scoglio" with its cruel leaps and two-octave range as the orchestral accompaniment tells us that she stands as a small skiff against a raging sea. These are Mozart's personal jabs at the operatic establishment, making fun not only of the characters' over-the-top emotions but of an art form that took itself perhaps a little too seriously.
In summary, it is best to try to listen beyond the exquisite beauty on the surface of Mozart's operatic scores to re-capture a sense of what it was that made the Da Ponte operas such a radical departure for the time. Yes, we see these very methods of musical communication in the operas of the great Christoph Willibald Gluck, a composer much admired in his day and beginning to get more recognition in our own. But Mozart went further and musically plumbed the depths of the psyches of his characters in a far more insightful way.