In order to create this balance between music and drama, composers aimed to move the plot forward and make the orchestra more of an important role in accompanying the vocalists. Rather than only playing simple harmonies under the vocalists, the orchestra now expressed emotions and moods pertinent to the opera’s plot. They also added choruses to many operas, something not common in Italian opera of that day.
Christoph Willibald Gluck was a pioneer of this new opera reform style. His Orfeo ed Euridice is an example of this. In this opera, Gluck uses the music to help further along the drama. He even uses a chorus as part of the action in the Chorus of the Furies of Act II. During this section, the orchestra certainly helps to convey the mood by using harsh strings, horns, and trombones to depict the Furies as Orfeo enters the Underworld. Gluck then uses a harp and softer, plucked strings to accompany the desperate Orfeo as he begs for mercy.
You can hear a clip from Gluck’s opera here:
How do you think this compares with operas earlier in the century?