Frying potatoes in cast iron: sticking!
Preheat the pan for longer than you expect, then preheat the oil as well. Cast iron takes a long time to heat up. With some foods you can get away with not preheating so it may seem silly to just let the pan sit on the burner with nothing else happening, but the potatoes will test your patience because of how much they can stick to pans.

Once you add the potatoes to the hot pan with hot oil, let them sit. They will go thru a stage of raw -> starting to cook -> cooked and starting to brown but sticking -> deliciously brown and movable from the pan.