This is probably the hugest, most diva-important solo written for English horn by any composer... ever.
I'm working on this excerpt for my sophomore proficiency, and I feel like I may have written about it a really long time ago when I first started working on it, but maybe not... plus I have more things to say about it now anyway.
Its dynamics and phrasing are probably the two things that make it so Wagnerian in nature... the dramatic contrasts and shapes really create a complementary sound to go with what is happening in the opera.
The prelude to Act III is the point when Tristan starts to go insane, and this solo English horn (which is played by the english horn player ON the stage btw) really triggers his downward spiral and helps to musically convey this to the audience.
The madness and anguish he feels is really represented (for me) through the triplet, slurred, eighth note patterns at the largo tempo incorporating large leaps and uncommon intervals. It gives what was originally a lyrical and nostalgic piece a sense of quirkiness, it most definately creates tension and strain which is usually resolved or at least momentarily relieved on the measures with the swarzando on the held note (usually a half note D or D-flat).
I feel like there is an overall structure even within a three and a half minute solo, there is the exposition which is developed upon during the section that I just discussed, with a return to the melody at the end of the piece, making it a sort of kind of little rounded binary.