The first problem that we are presented with when listening to Don Carlo is that we have to make a decision as to which version to study: the original, five-act French version written for the Paris Opéra, or one of the three Italian versions that were later authorized or 'devised' by the composer himself. For our purposes, it is important to know that our company has decided to produce the four-act Italian version, but for listening at home I can't imagine being without a recording that incorporates at least most of the Fontainebleau scene, the original Act I. In production, often a director will pick and choose between the various versions and come up with a 'practical edition' that works for the stage. Just understand from the outset that dealing with Don Carlo is no easy feat and you, the listener, must decide which version you'd like to spend time with.
Musically speaking, the first, French version of Don Carlos marks the end of Verdi's fruitful middle period (La traviata, Rigoletto, Il trovatore, Simon Boccanegra, etc.) but his 1882-83 revision places it at the beginning of his late period, after Aida but before Otello. It should be remembered also, as The New Grove Dictionary of Opera points out, that there is no true Italian 'version' since Verdi always worked from the original French text; there are only Italian translations.
Musically the work is weighty and dark, matching the color of the text and the quasi-historical situations perfectly. The orchestration favors the darker colors of the orchestra, lower strings and winds as well as brass when needed. There are standard forms, certainly, arias, duets and other ensembles that seem to be indicative of a traditional mind at work. But these numbers are so tightly woven into the fabric of the drama that we hardly notice them. The most interesting of these so-called numbers are the duets which propel the action forward: Carlo and Posa's duet in Act I, scene 1, Carlo and Elisabeth's duet in Act I, scene 2, Philip and Posa's duet at the end of the same scene, Philip and the Grand Inquisitor's duet in Act III, scene 1, Carlo and Posa's duet in Act III, scene 2. All of these duets break with traditional duet structure to a great extent, and are among the most musically convincing examples of Verdi's dramatic style.
We need to remember when experiencing Don Carlo on CD or in the theatre that the piece was written for a French audience: it had to live up to the expectations of that audience and fulfill their lyrical demands, as well as their demand for spectacle. In Don Carlo, Verdi could take time to tell his epic story. Still, there is an unparalleled sense of dramatic urgency in the work. Now, he needed only to compress his style and to be more economical with his lyric gifts in order to achieve the greatness that is Otello and Falstaff, works that share that same sense of urgency but that tell their stories more directly and with more immediacy.