The trill, like scales and cadenza work, is very much a question of a correct ear.
I mean to say that a person who has a very good ear, naturally or through training, gets it very quickly, while the singer who is less talented musically, must strive and work for it, and depend more upon his teacher.
I said before that the trill is a tremolo — a perfected, voluntary, controlled tremolo.
In the vocal trill the accent must be on the upper note.
Now, Trilby, here is the great preparatory exercise to the trill. It is one handed down direct from the masters of the old Neapolitan School:
There, just practise that in one octave in the center of your voice. (Light soprano, A to A; dramatic soprano and mezzo- carratere, G to G; mezzo-soprano and contralto, D to D; tenor, G to G; all baritones, D to D; basso, B to B.)
This exercise must be practised twice daily for a couple of weeks, and then we can take it in this way:
To trill D and E, the tremolo must be made on the E — it must be a tremolo that descends a whole tone on every vibration.
Your thought must go to the E as soon as you’ve struck the D and remain there.
Ah, yes, Trilby, you can understand because I can show you — I am here with you, but unless my pupils who are under going this absent treatment have very good ears, it will be hard for them.
The trill is the most difficult part of vocal technic to teach without illustration. That’s why Sbriglia couldn’t teach it when he was so awfully old that his own voice wouldn’t do it any more, and he was pretty deaf, too, during his last years.
The trill must be practised in the beginning within a very limited range.
It usually comes first on the lower tones.
A soprano trying it from G to G, for instance, should trill the first five notes of the scale to begin with: G and A the whole tone, then A and B whole tone, then B and C half tone, then C and D whole tone. When these notes begin to go clearly the D and E can be added — say after a few weeks — and gradually up the scale. When the different voices can trill clearly in the ranges I have already indicated for each, they can be pretty well satisfied, anything further being exceptional and of no real importance, except in the case of the higher voices of women.
Duval, John H. “The Secrets of Svengali on Singing, Singers, Teachers and Critics.” (1922).