I am posting part of an answer from another response here that I'd written because it seems applicable to understanding the Chorus' growth:
I am writing my dissertation on John Steinbeck, whose observations about group behavior apply to that of Sophocles' Chorus.
Steinbeck noticed early on the tendencies of groups to act differently when together than they do when acting as individuals. You might know this as "mob mentality" but Steinbeck argued that animosity need not be a factor in altering behavior; just the assembled of many persons affects their psychology. He called this theory "group-man" and later "phalanx theory."
I think this is what happens to the choral members. They move, think, speak as one. Yet, when the leader is allowed his own asides, he sees more clearly, his advice is more sound; he is not lulled by remembrances of things past (Proust, of course), his voice is not blended among those of his peers.
Once the leader changes his opinions, the group members likewise are swayed. They may have come to such conclusions on their own, but as a group, they would not have done so without some outside influence (the leader stepping out and telling them, "now we think this.")
What do you think?
The Chorus seems to change the most during and after Haemon's confrontation with his father, Creon. Haemon is very careful at first, flattering his father with words of praise and honor as a dutiful son should, then slowly turning it around to let his father know that things aren't all that great in Thebes these days. He lets Creon know that the people of Thebes (not himself) are upset by his decision concerning Antigone, which causes the Chorus to tell Creon to listen to Haemon:
"If he says aught in season, heed him, King." (They do then remind Haemon to listen to his father, too.)
Perhaps it is the turning of the people against Creon (according to Haemon) which begin to change how the Chorus looks at Creon. Their concern is for the city of Thebes, and now that things are looking rather unsettled, they begin to caution him about the steps he's taken.