Mozart's Beginnings

“How does Mozart get our attention in a musical number, that is, how does he begin his music in a dramatically, meaningful way?”

In all vocal music, not just the works of Mozart, the text and the music are inseparable.   In opera in particular, the driving force behind recitative and arias are the characters needs and desires.   Just as in real life, what the character is saying may be very different from what he or she actually wants.   Mozart’s understanding of emotions is clearly portrayed in his music.   The way he starts a piece is very important; as an actor, one can draw artistic direction from the orchestra’s music that brings in the vocal line.   An example of his dramatic intention through the music at the beginning of a piece is Susanna’s recitative and aria in Act IV of Le Nozze di Figaro: “Giunse alfin il momento...Deh vieni non tardar.”
The recitative begins in C Major, Allegro vivace assai, with the Violin II’s establishing a steady eighth note foundation that outlines the harmonic changes.   The harmonic rhythm changes every measure, giving Susanna the necessary appearance of nervousness and excitement.   The opening to the recitative shows that Mozart wanted Susanna to play this scene a certain way (or at least giving her the option to do so). Mozart uses the lively, almost jumpy texture of the orchestration to signify that Susanna is sneaking through the garden, nervous to meet the Count or be discovered.   “The moment finally arrives when I’ll experience joy, without haste, in the arms of my beloved!”  
Figaro, who hides throughout this entire scene, surely feels pangs of betrayal as Susanna voices her “desire” to be with the Count.   Susanna, surely, may be “timid” and “fearful,” but it is not for the reasons Figaro believes.   Most likely, Mozart wanted Susanna to dig in and make Figaro believe she wasn’t faithful, but she cannot help herself when she talks about love: to her, it is all about her husband.  
The aria begins in F Major, with...