"The tone of the voice was warm, lyrical, intense; it swirled and flared like a flame and filled the air with melodious reverberations like a carillon. It was by any standards an amazing phenomenon, or rather it was a great talent that needed control, technical training and strict discipline in order to shine with all its brilliance."

In 1968, Callas told Lord Harewood:

"De Hildalgo had the real great training, maybe even the last real training of the real bel canto . As a young girl—thirteen years old—I was immediately thrown into her arms, meaning that I learned the secrets, the ways of this bel canto, which of course as you well know, is not just beautiful singing. It is a very hard training. You have to learn to read, to write, to form your sentences, how far you can go, fall, hurt yourself, put yourself back on your feet continuously. De Hidalgo had one method, which was the real bel canto way."

De Hidalgo later recalled Callas as "a phenomenon... She would listen to all my students, sopranos, mezzos, tenors... She could do it all." Callas herself said that she would go to "the conservatoire at 10 in the morning and leave with the last pupil ... devouring music" for 10 hours a day. When asked by her teacher why she did this, her answer was that even "with the least talented pupil, he can teach you something that you, the most talented, might not be able to do."

Matthew Gurewitsch: "In fact the essence of her art was refinement. The term seems odd for a performer whose imagination and means of expression were so prodigious. She was eminently capable of the grand gesture; still, judging strictly from the evidence of her recordings, we know (and her few existing film clips confirm) that her power flowed not from excess but from unbroken concentration, unfaltering truth in the moment. It flowed also from irreproachable musicianship. People say that Callas would not hesitate to distort a vocal line for dramatic effect."