[...] I was very much prepossessed in your favour by your talented performance in The Flying Dutchman which I witnessed, and I assure you that I see no task even in my great tetralogy which I am not convinced you could accomplish excellently. I have, however, assigned you the most difficult rôle in every respect, one which it gave me great anxiety to cast, namely that of Alberich, a part filled with daemonic, passionate tragedy. The part runs through three of the pieces, The Rhinegold, Siegfried and The Twilight of the Gods. I can hardly wish you to go to the trouble of studying it in advance as I should prefer to introduce you to it myself, feeling that I am the only person competent to do so. Still I should be glad if you would look into the poem beforehand. [...]
Source: Letters of Richard Wagner, Vol. 2, selected and ed. Wilhelm Altmann, trans. M. M. Bozman (J. M. Dent & Sons, 1927), No. 668