God's Timing Is Weird

God's calling and filling does not come in expected ways or obvious times.  In 1 Samuel 16, how odd that God chooses David, fills him with his spirit, yet the beginning of his kingship lies several years ahead.  Of course David is young at the time.  Saul has messed up badly.   Still God knew from the beginning that David was his man.

Why involve Saul at all? Why fill young David then have him wait for the fulfillment?

Why fill one man and leave another not neutral, but with a spirit of fear/despair?

James Tissot, Jesse Presents His Sons to Samuel 1900