Describe the structural elements which stand out in the first scenes of Act 1. How is musical improvisation used as a literary technique?

One important point to note about this section is how comments in one scene and/or song reflect comments in another - for example, the doctor "improvising" with Diana's medications related to Natalie and Henry's discussion of musicians "improvising". This narrative technique, of parallel/mirror imaged meanings is one which theater often makes very effective use of.