The promise is barely given when famine hits, and Abram's first response to crisis is fear, not faith. He tells Sarai to say she's his sister — technically true, since she's his half-sister, but built to deceive — and Pharaoh takes her into his household. God intervenes where Abram wouldn't; Pharaoh, a pagan, ends up behaving with more integrity than the patriarch, rebuking him and sending them off enriched.
Here's what's worth actually seeing, though, structurally: famine drives them to Egypt, a plague falls on Pharaoh, and they leave enriched and set free. That is the Exodus, in miniature, four hundred years early, run first through one man's private failure before it becomes the nation's public deliverance. Abraham's fear rehearses Israel's later story. And don't let this episode go without noticing that Abraham does the exact same thing again in chapter twenty, with Abimelech, decades after his own justification in chapter fifteen — and Isaac repeats the same lie about Rebekah in chapter twenty-six. Three generations, the same failure. That repetition matters more than any single instance of it, because it tells you sanctification doesn't move in a straight line, even for the man Scripture calls righteous by faith.