[...] It is high time I wrote to you! First of all, my sincerest thanks for your protracted efforts with Unger. The good results are quite striking. I find that his voice has become very much more true, and that accurate study of all and sundry has stood him in good stead. After a careful study of the organic peculiarities of his voice, however (peculiarities which proclaim themselves, even when he speaks, as a natural constriction of the throat), I have decided that he must always sing with the full power of his voice, as then the fatal palatal sound disappears from high notes and low, but particularly from the latter. On this system he must rely on warmth of emotional expression (a certain vibrant veiling of the tone) to get his gentler effects, but this will only emphasise the whole character of Siegfried, from whom nothing is more alien than lyrical sweetness, and who is distinguished by daemonic high spirits and impetuous self-confidence. [...]
Source: Letters of Richard Wagner, Vol. 2, selected and ed. Wilhelm Altmann, trans. M. M. Bozman (J. M. Dent & Sons, 1927), No. 690