Chopin wrote:What kind of training should an interpreter have? How can one develop the ability to have one's own say on a
work? Even an unpublished one?
A great starting point, truly. And if we manage to continue along this track, we might also succeed in clarifying the very important role of critical editions and the difference between them and others.
I believe it is fundamental, in order to "have one's own say" on a piece, to be familiar with the work of the author of that piece. Whether it is published or unpublished, the point is the same, in my opinion, namely: the more I know about that author, the more elements I will have to approach their work. A friend who was a conductor at the beginning of his studies, some time ago, asked me which Beethoven symphony he should study, in my opinion. I replied that he should start with whichever one he wanted, since one must know them all! Obviously, my point was paradoxical: starting with the Ninth would have been nonsense, especially because, to deeply understand the greatness of that symphony, it is nevertheless necessary to know what Beethoven wrote before. It is natural, therefore, that the discussion is not solely and exclusively aimed at performance: I can also decide to conduct only a few pieces in public, but to fully understand those pieces, I must also know the others.*
I was speaking of "familiarity with the author's work," and it is precisely here that the work of editors comes into play, providing us with texts derived from sources, cleaned of posthumous interventions, cleaned of errors that have persisted over time and, above all, cleaned of interpolations by other musicians, whoever they may be.
In the 19th century, Breitkopf (I am using an example I know very well) published an immense amount of music; practically all German composers from Bach onwards were published by Breitkopf. The work on manuscripts or first editions was certainly not as accurate as what is done today, and not only were printing errors from the first editions transferred into the new Breitkopf editions, but they even "corrected" presumed errors made by the composer, without bothering to verify whether those "errors," within the style and language of that composer, were not actually characteristics. And it was thus, for example, that from the music of Beethoven, Mozart, Brahms... wedges disappeared, which automatically became dots (but both Mozart and Beethoven and Brahms used both with great accuracy, depending on the case!); it was thus that many of Schubert's accents magically became diminuendo hairpins (a striking case: the "Great" Symphony, which ends with a long C for the whole orchestra, fz and with an accent >, which became a diminuendo for the simple reason that Schubert, having to accent the long note, had also lengthened the accent!).
And this is just to mention a couple of details (which are nonetheless fundamental, especially if you think of them being transferred to the entire instrumental production of these authors), and to remain silent about a myriad of more or less serious errors, wrong notes, standardized phrasing, shifted colors, etc.
If you listen to recordings from the '50s, '60s, '70s, made by great orchestras and great conductors, you will realize that these errors exist, but naturally it is absolutely not the fault of the musicians: the responsibility lies with the publishing houses, which until a certain point held an exclusive on publication and continued to disseminate "mis-corrected" materials. Today, fortunately, it is no longer so, and the use of critical editions by musicians is increasingly widespread. Of course, the observation that one can find 19th-century editions full of inaccuracies on IMSLP and that this compromises accurate work is correct, but it is also true that awareness of what is correct or not is becoming increasingly widespread, at least in professional circles.
Why did I make this point once again? Because to the question of how one can be able to "say something" and my answer regarding "familiarity" with one author or another, I could not help but follow up with some observations regarding the only way I believe is possible to acquire familiarity with an author, which is to study them using a text that has been carefully revised in a critical sense by competent musicologists and attentive scholars.
Years ago, I studied Beethoven's Sonatas in the Henle edition (Urtext), on whose title page it is written: NACH EIGENSCHRIFTEN, ABSCHRIFTEN UND ORIGINALAUSGABEN - HERAUSGEGEBEN VON B. A. WALLNER, FINGERSATZ VON CONRAD HANSEN (that is: from manuscripts, copies and original editions - edited by..., fingering by...). Faced with an edition of this type, one knows from the beginning that the work has been carefully curated based on the declared sources, that the fingerings are not Beethoven's (except for those marked in italics, autographs, rare but present - see e.g., the passage of glissando octaves in}
Waldstein, fingered by Beethoven himself) and that everything which has been "added" is based on those sources (that is, on customs found in LVB's style) and is marked with a different character, or in parentheses, or in a note. The same applies to all other authors, published over the years by both Henle and Bärenreiter, the two reference publishing houses for (almost) all music, along with Boosey & Hawkes for Russians and Americans.
When I started studying, after a few years, my familiarity with Beethoven's symphonic repertoire came from there, from the Sonatas, and I recognized a style, a custom in using certain signs etc. It is to this type of education that I was referring a few posts back, when I maintained the great importance of using, from the very beginning, texts curated in this way, to avoid becoming accustomed to editions accompanied by additional and entirely personal indications, which only serve to confuse the ideas of those who do not have full awareness of what should be considered authentic and what should not. And the Breitkopf editions I referred to before, but also all the Ricordi editions of Beethoven's Sonatas, of Bach's music, of Mozart... everything in short, in no way indicate the interventions of the "editor", leading the unsuspecting student to think that everything written on the paper is authentic and, in any case, preventing them from distinguishing what is and what is not.
* We know, for example, that ABM knew an enormous amount of repertoire by heart, even if he then chose to perform only a few pieces in public. The same applies to Kleiber, who conducted very few symphonies in public and even fewer operas, but knew an unthinkable amount of music.