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For me personally, one of the key themes in Emperor and Galilean, was friendship. The strong bond between Julian, Gregory, Peter and Agathon.
Julian loses his way over the course of the production because, one by one, starting with Gregory, his friends leave/desert him. His "good advisors", who have his best interests at heart, leave him - his support networks and the firm foundations on which his character are built fall away. He ends up relying on Maximus, who is not a friend and therefore not a great advisor, and starts to believe in his own divinity, immortality and invulnerability. He starts to make ill informed decisions and makes bad judgements. He renounces Christianity and takes up Paganism, relying on signs and omens. That is the end of him!
This play shows the importance of good friends. They will always have your best interests at heart and will advise you well. Never forget that in times of need!