CDs and videos of famous singers giving lessons are often not very interesting.
The CDs of Maria Callas' master classes just consist of her singing and expecting the student to imitate her. We don't need a master class for that--you can do that by playing recordings of her singing.
I'm not sure imitation is a bad thing. There may be no other way to achieve the right style for Italian opera, for instance. It's good enough for Luciano, it may be good enough for you, too.
I bought Grace Bumbry's dvd "Homage a Lotte Lehmann." Lotte would have done a better job with most of these pieces. Grace is taking herself much too seriously.
The dvd alternates songs with talking, and in one section toward the end she complains that most of the students she meets are not ready for interpretation. This makes my teeth grind. No interpretation is also an interpretation. I suppose they're supposed to sing every note in a kind of flat dynamic until they get everything perfect. Every vowel, every rhythm, every note. When they've achieved a kind of school book perfection we'll let them think about interpretation. Ugh! Why would they bother? Why wouldn't they just faint from overwhelming boredom? I would far rather they looked for meaning every second and had to be calmed down. Have something to say or give it up.