In the time of Corneille's CINNA, incidental music was often performed between the acts of plays. If there are just 5 movements of incidental...

Generally speaking, incidental music would have been played before each act. Therefore, the five separate movements of incidental music would have been played before the five acts of Corneille's puppet-play Cinna: one movement before the first act, one before the second act, one before the third act, one before the fourth act, and the fifth movement before the fifth act.

Given that Corneille's Cinna is in 5 acts, the performance would likely have started with Harrison's...

Generally speaking, incidental music would have been played before each act. Therefore, the five separate movements of incidental music would have been played before the five acts of Corneille's puppet-play Cinna: one movement before the first act, one before the second act, one before the third act, one before the fourth act, and the fifth movement before the fifth act.

Given that Corneille's Cinna is in 5 acts, the performance would likely have started with Harrison's first movement.  Herein Harrison set a "mood" for the entirety of the play and set up the introduction of motivations, as illustrated by Harrison's first movement dynamical notes, which start with "medium fast, medium soft" and move to "grow louder" then jump to "suddenly soft."

This procedure of playing music before each act is still used today. The second, third, fourth and fifth movements build organically out of the first in such a way that the first gives a foretaste of what the other movements of the incidental music will hold while the listener is given a hint as to the various moods of the entirety of the piece.