The icon must depict externals, but also interior life, holiness, and proximity to Heaven. The principal means for accomplishing this is in the face, the facial expression and its look depicted on the icon; also, the rest of the icon must be consonant with that expression. It was on the depiction of the inner condition of the soul, hidden by the flesh, that our Orthodox iconographers focused their attention. The better they were able to accomplish this, the better the icon appeared to be. Often, there would be deficiencies in the manner of depicting various parts of the body – not because iconographers were doing it consciously, but because [their efforts to] accomplish their principal goal did not always allow them to give sufficient attention to secondary aspects of their work.
Elder Joseph the Hesychast in Monastic Wisdom.