1 Samuel 18:10

And it came to pass on the morrow
After the women had met him with their music and dancing, and when returned home:

that the evil spirit from God came upon Saul;
thinking on the above things that had passed, he became melancholy:

and he prophesied in the midst of the house;
either really, delivering out divine songs, as the prophets did; according to Abarbinel, he foretold that David would be king, and the kingdom would be taken from him, and given to him; or he feigned himself a prophet, mimicking their motions and gestures; or, as the Targum, acted like a mad man, or a fool, uttering foolish words, and using ridiculous gestures, which seems most agreeable to the evil spirit in him:

and David played with his hand as at other times;
upon his harp, to remove the evil spirit, or melancholy disposition from Saul; for though he was now advanced at court, and an officer in the army, and high in the affections and applause of the people: yet he did not think it below him to act as a musician, to do service to his prince; of such an humble, kind, and ingenuous disposition was he:

and [there was] a javelin in Saul's hand;
a kind of spear, or half pike, which he had taken into his hand on purpose to kill David while playing; for persons in such circumstances as his, as they are very mischievous, so very subtle at contriving.