We really hear a woman fearing for her whole being. But being able to sing it through in this manner is another thing, and Watson manages it. That is the magic of Mozart's vocal music. Composers like to see it as his having written certain unobtrusive things into his vocal lines that make it impossible for second rate singers to sing them, at least not without exposing their vulgarities and showing how bad they are. This aria has all of those things in spades.
Few singers can really control themselves through the long, slow phrases.
They have all they can do just vocalizing it, let alone singing with subtle expressive nuance up there in that high tessitura. Well, Watson gives us all a singing lesson here, and Klemperer gives us a lesson in concertante accompaniment of the highest refinement and musical culture. There isn't a single note that she doesn't give full, subtle, delicately delineated emotional content. Other singers are happy just to get those notes out, she phrases them completely, begging God for pity, and giving the 7th and 8th sixteenth a completely different emphasis and inflection from the preceding notes that no one else manages. That is Klemperer in spades.