His gleaming baritone voice made a huge impression on me when I was in graduate school. Though I have to say; the first time I heard Gérard Souzay, it wasn't a revelation as much as it was a window into a world that was. Frankly speaking, when I heard Souzay at a summer Art Song Festival concert at Westminster Choir College in Princeton in the 1980's, it was evident that his voice was in decline. It was only when I later listened to an LP of Debussy songs that I realized why everyone was making such a fuss: Souzay was simply the most elegant singer of his time. What a beautiful voice! His mezza voce singing in the upper range made my hair stand on end. How did he do it? It was only much later that I learned that the secret in acquiring this most beautiful of vocal effects was in practicing messa di voce, first in a comfortable lower range, and then in the higher more challenging range.
It's systematic you see. You have to able to do one thing before you can do another. Speaking of which: how does one practice messa di voce? Manuel Garcia taught his students that the way to go about the matter was the artful decrescendo of a full ringing tone without losing any quality. Then one learned how to start the tone with this same quality and increase the volume. Join the two 'halves' of your practice together, and you have messa di voce.
If anything, Souzay is exemplary in his shaded, soft singing. You can listen to his art below, or watch him in action here. He sings Duparc's Chanson triste in the latter link - a song that still gives me goose bumps.
One more thing about this beautiful singer which is quite evident in the Duparc: Souzay uses his right ear to lead his voice.