1 Samuel 21:15

“Have I need of madmen? etc.] Or fools, do I want them? have not I enough of them already at my court? I want wise men, and not fools and madmen. The Jews say f448 that the wife and daughter of Achish were mad; that while David was playing the fool and madman without, they were acting the same part within; so that Achish had enough of that sort of diversion, if it was to be reckoned such; as it was according to the taste of some persons, who used, as in later so in earlier times, to keep fools in their houses to make them sport; but Achish had enough of that, and too much, at least needed no more: that ye have brought this [fellow] to play the madman in my presence? or act the part of a fool…”

“Mad men - It is highly probable, Achish was aware, that this madness was counterfeit. But being desirous to preserve David, he speaks as if he thought it real. Next: 1 Kings (1 Samuel) Chapter 22”