And it came to pass on the morrow, that the evil spirit from God came upon Saul, and he prophesied in the midst of the house: and David played with his hand, as at other times: and there was a javelin in Saul's hand. The evil spirit from God - See on 1 Samuel 16:14 (note), etc.
He prophesied in the midst of the house - He was beside himself; made prayers, supplications, and incoherent imprecations: "God preserve my life," "Destroy my enemies," or such like prayers, might frequently escape from him in his agitated state. The Arabic intimates that he was actually possessed by an evil spirit, and that through it he uttered a sort of demoniacal predictions.
But let us examine the original more closely: it is said that Saul prophesied in the midst of his house, that is, he prayed in his family, while David was playing on the harp; and then suddenly threw his javelin, intending to have killed David. Let it be observed that the word ויתנבא vaiyithnabbe is the third person singular of the future hithpael; the sign of which is not only to do an action on or for one's self, but also to feign or pretend to do it. The meaning seems to be, Saul pretended to be praying in his family, the better to conceal his murderous intentions, and render David unsuspicious; who was, probably, at this time performing the musical part of the family worship. This view of the subject makes the whole case natural and plain.