In an earlier post, I wrote about frying potatoes in the same way as you would chips. The only real difference from chips is that these are cubed.
The second method, which I tried the other day, is to parboil the potatoes first. You cube them, cover them in cold water, bring them to the boil, and simmer them for about 10 minutes. They need to be soft, because frying in a thin layer of oil does not tenderise them much further; but if they are too soft they will break up. Drain them, and return them to the hot pan to steam. I think that there is a case for sprinkling them with flour, to add crunch to the surfaces. (I do not think that is necessary with roast potatoes.)
Warm a layer of sunflower or other oil of your choice in a frying pan. (As I wrote earlier, you will need a 28 cm pan to accommodate four medium-sized potatoes.) Tip in the potatoes, and cook them on a medium to low heat. It may take 10 minutes or longer for the undersides to brown. Turn them, to get them as thoroughly brown as you like.
In my last entry on roast potatoes, I wrote that you could turn the parboiled potatoes in cold oil, and get perfectly crispy results. Why do I not say that here? Because these potatoes have been parboiled for longer, and are softer. They might break up if you agitate them a lot; and they will absorb more oil. Tipped into hot oil, they will form a crust more rapidly.