Charles Spurgeon Commentary 1 Samuel 18:10

"And it came to pass on the morrow, that an evil spirit from God came mightily upon Saul, and he prophesied in the midst of the house: and David played with his hand, as he did day by day. And Saul had his spear in his hand;" — 1 Samuel 18:10 (ASV)

And it came to pass on the next day, that the evil spirit from God came upon Saul, and he prophesied in the midst of the house:

Probably, talking wildly and foolishly.

And David played with his hand, as at other times:

That is, as he had, at other times, played upon the harp to chase away the evil spirit.