Some luck, some skill. It matters where and how the line and bait is cast, what size is the hook, how much crap is in the water that your line gets caught in/on, and how your bait is ensnared on the hook. This is assuming you have bait or a lure that is bite worthy to a fish. If a fish hides behind a big rock across the river, and the river is running swiftly, it helps if the fisherperson can cast well enough so that the lure ends up, at some point, just behind that rock.