The song form of Tin Pan Alley that is tied to a particular style of performance art (musicals) is 32-bar form, also known as as the AABA song form, American popular song form and the ballad form.
"Over the Rainbow" fits into the ballad or 32-bar form.
The orchestra plays the role of a pit ensemble or pit orchestra for musicals. It accompany vocalists to evoke emotion into their performance.
