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The significance of the line: "He wears a mask and his face grows to fit it".
What is the narrator's feelings about the role he plays in being part of the colonial government in Burma. In fulfilling his position as a member of the police force, he is required to be someone he is not and to enact politices with which he does not agree.
He feels imprisoned himself by the political and social forces at work in the colony.
Other formulations may be accepted :)