A little clarification. One way to define the artistic limits of 19th century Italian opera is by the way works were cast. New operas were commissioned on a commercial basis. An impresario would rent a city’s opera house, recruit singers, and find composers to write operas for the singers to sing. The singers fell into established types — a prima donna, a primo tenore, a baritone, maybe a buffo or serious bass, and a few comprimarios, singers of small roles, who clearly weren’t very good.
These comprimarios, and their apparent lack of any real ability, set limits on what opera could be. An opera like a Dickens novel, full of distinct and memorable characters, was impossible. The comprimarios just couldn’t handle it (nor would the audience, looking for romantic thrills from the leading roles, be interested, but that’s another story). That’s why Blind Alice, a memorable figure in Sir Walter Scott’s The Bride of Lammermoor, turns into Alisa in Lucia di Lammermoor, a typical mezzo-soprano confidante with hardly anything to do, and without a trace of individuality. (Fun fact: Alisa’s most notable moment in the opera comes in the stretta of the second act finale, when she has a series of solo screams on high A — in a passage that’s usually cut.)
Even the leading roles tended to be types — the doomed romantic heroine, the equally doomed romantic tenor adventurer, the regal or priestly bass, the wronged baritone husband. A composer lucky enough to find a striking actor or actress in his cast could write something striking, a role with depth, like Norma or Lucrezia Borgia or Rigoletto. Sometimes operas even had a wide array of roles, as Donizetti’s Lucrezia Borgia does, but, though the minor characters are numerous, it’s hard (as it is in Rigoletto) to tell them apart. Sometimes operas attempted to portray something other than romantic adventure or aristocratic life. Donizetti’s Linda di Chamounix has a promising array of rural characters, as does Verdi’s Luisa Miller.
But again these operas were exceptions, and what you’ll almost never see is a bourgeois figure, a member of the middle class, even though this was the most notable and fastest growing part of society during this period. Germont, in La Traviata, is a notable exception. Opera, in those days, just didn’t deal with real life. It was extravagant entertainment, full of what, from a middle class point of view, were Others — aristocrats, outlaws, and peasants (usually idealized). In the last part of his career, Verdi started to create figures outside this box, like Sam and Tom, the sardonic conspirators in Ballo in Maschera, or Fra Melitone in Forza. Melitone and Falstaff might be in some ways Verdi’s most notable characters, or at least the ones that jump outside the frame of what opera characters usually were. Both come from the people; both are, in their way, almost Dickensian.
A whole opera of Dickens characters, though — that was impossible, even though Dickens novels were routinely adapted for the non-musical stage. Even more impossible would be the range of bourgeois types so slashingly portrayed in Madame Bovary. The title character has her operatic moments, but the clueless husband, the shallow lover, and above all the leading figures in the village all are both too individual and too improper for opera of that time. And for all his depth in adapting Shakespeare, Verdi wouldn’t have been able to handle lots of things in Shakespeare’s plays — the bawdiness, for instance, and the full range of everyday characters. Characters like Bardolph, Pistol, and Dr. Caius (all from Falstaff) are barely sketched in, mere shadows of what they are in Shakespeare.
20th century opera composers learned to do what the 19th century couldn’t. Composers like Britten (think of the range of village types in Albert Herring) or Janacek (unforgettable cameos in The Makrapoulos Affair) or Berg (the entire casts of Wozzeck and Lulu) learned to do what novelists and playwrights had been doing long before. But 19th century opera, for all its power within its limitations, remained a restricted artistic genre, in some ways more like popular entertainment than serious art. (Or, maybe better, a striking example of the ways art and entertainment can blend, with each side making compromises.)