OK, I have never personally witnessed it (that I know of), but I have seen movies and understand what demonic possession is. But what do you mean by "perfectly" demonically possessed?
When there is "perfect" demonic possession, then the demon has total control of the body, mind and will of the person. "Im-perfect possession" would mean that the demon is influencing the person, but the person still has control of his will.
That's my recollection from reading "An Exorcist Tells His Story" by Fr. Gabriel Amorth. Highly recommended reading! An outstanding work of theology. I was afraid it would be mostly sensational anecdotes, but it was nothing of the sort. His explanation of the presence and influence of demonic spirits was sobering and enlightening.