Albert Barnes Commentary 1 Samuel 28:2

"And David said to Achish, Therefore thou shalt know what thy servant will do. And Achish said to David, Therefore will I make thee keeper of my head for ever." — 1 Samuel 28:2 (ASV)

You shall know ... - David dissembled (compare also 1 Samuel 29:8), hoping, no doubt, that something would happen to prevent his fighting against his king and country.

Keeper of my head - Captain of his bodyguard.