What is vocal placement? Let me see if I can explain this as simply as possible.
Correct voice placement consists of what is known as singing in the mask; in other words, the frontal cavities of the face and the upper front palate. The tone produced must be steady throughout the singing without slipping in and out of position – almost like a laser beam in that it’s totally focused.
A perfect example of uneven singing is Lanza’s Coca-Cola Amor Ti Vieta. The beginning of the aria is out of position, as it's crooned and not sung mezza voce. When singing mezza voce the position must remain the same, but the volume is obviously reduced. Mario is in position on “Di non amar,” but falls back in “Lieve,” “Respinge,” “Cerca la stretta,” “Non t'amero,” etc. Listen to the much-superior 1955 version for a comparison. The same applies to the Coke Flower Song compared with the commercial version. Listen to the two renditions and you'll hear the difference. It makes for very uneven singing, typical of some of Lanza's Coke Show recordings. In order to maintain a constantly even, focused tone, it's not enough to do a few scales, run through the number once or twice and then sing it. What is required is regular daily work with a competent coach, something that was obviously not happening during the Coke sessions – hence the erratic output.
Basic vocal production will obviously alter according to the role being sung; obviously, you are not going to sing Nemorino in the same way as, say, Manrico.
Correct voice placement consists of what is known as singing in the mask.
Wagnerian singing requires more power and a broader sound, as does Del Monaco's type of singing, or any of the Melocchi school of singers, but they all sing in the mask – after all, that's what technique is all about. From the early 60s, Del Monaco tried to lighten his emission and that resulted in an ugly nasal sound. Of course, the opposite is the lack of technique displayed by both Di Stefano and Carreras, particularly in their later years. There's no mask or covering at all; the singing is open, back in the throat and consequently the high notes from A up are lacking resonance, squillo, and are in fact, screams. A voice that's not placed very forward will always sound more beautiful than one totally in the mask, as, for example, Domingo, whose sound – particularly in the middle register – is quite nasal.
Lanza was a case apart. Gifted with an exceptional voice, he was able to sing in the mask, cover and vary his vocal production with only a hint of nasality on one or two occasions, such as “My moonlight SERENADE” in The Student Prince.
As for French singers, I attribute their excessive nasality more to the language, which lends itself to nasal sounds probably more than any other. As Callas said, "The perfect language for singing is Italian."
What about singers such as Di Stefano and Carreras, whose vocal glory days were relatively limited?
Well, at the start of his career, Di Stefano had a basic technique, which he had acquired from his teacher Montesanto. However, he refused to continue along the lines that Montesanto was insisting upon, saying that it was bottling up the voice and that he couldn't sing with the voice "impostata" meaning in the mask.
Gifted with an exceptional voice, Lanza was able to sing in the mask...
Montesanto told him that if he continued to sing open and not cover, he would be finished in five years. A prediction that unfortunately came true as by 1952 – with the addition of heavier roles such as Cavaradossi – the voice was already showing signs of wear in the upper register.
Much the same can be said about Carreras, who never really learned to cover in the passaggio, and who (like Di Stefano) took on heavier roles far too early. As a consequence, two of the most beautiful tenor voices to emerge in the last sixty years had careers that, at most, lasted a mere ten years at the top.
As for the reason most singers take on heavier roles prematurely, it's because there's more satisfaction in singing the big romantic/dramatic parts as opposed to the lyric/ light lyric ones. The other reason is that that's where the big money is!
Impresarios and agents don't care a fig how long a singer lasts: they will use and squeeze him or her for all they are worth and then dump them in favour of a new lot. Few have the willpower to stick to the right repertoire. One example is Alfredo Kraus. But if you as an opera-goer had a choice, who would you rather go and hear, Kraus singing Ernesto in Don Pasquale, or Di Stefano in Andrea Chenier?