I've been reading some of the visions of Ellen G. White - the 19th century Seventh Day Adventist prophet. She gives a fascinating account of the War of Heaven - with this description of the fallen angel:
I was shown Satan as he once was, a happy, exalted angel. Then I was shown him as he now is. He still bears a kingly form. His features are still noble, for he is an angel fallen. But the expression of his countenance is full of anxiety, care, unhappiness, malice, hate, mischief, deceit, and every evil. That brow which was once so noble, I particularly noticed. His forehead commenced from his eyes to recede. I saw that he had so long bent himself to evil that every good quality was debased, and every evil trait was developed. His eyes were cunning, sly, and showed great penetration. His frame was large, but the spiritual flesh hung loosely about his hands and face.
As I beheld him, his chin was resting upon his left hand. He appeared to be in deep thought. A smile was upon his countenance, which made me tremble, it was so full of evil and satanic slyness. This smile is the one he wears just before he makes sure of his victim, and as he fastens the victim in his snare, this smile grows horrible.