Charles Ellicott Commentary 1 Samuel 28:20

"Then Saul fell straightway his full length upon the earth, and was sore afraid, because of the words of Samuel: and there was no strength in him; for he had eaten no bread all the day, nor all the night." — 1 Samuel 28:20 (ASV)

Then Saul fell straightway all along on the earth. —Until this point, we must understand Saul listening to the prophet’s words in that attitude of humble reverence which he assumed when he perceived that he was in the presence of Samuel (1 Samuel 28:14); but now, on hearing the words of awful judgment, crushed with terror and dismay, and previously weakened by a long fast and the fatigue of the rough night walk from Mount Gilboa to En-dor, he fell prostrate to the earth.