If you are not parboiling the potatoes first and using them raw, soak the cubes in water for 15 minutes. They will turn out crispier and get rid of the extra starch.  If you are parboiling them first, add little salt and turmeric when doing so. This ensures the flavor and color get infused into them. Remember, in the end, the potato cubes must hold their shape and not become mushy. Do not overcook them. Leave the skin on if needed, to tap in its nutrients.  Do not cover the pan while cooking and add more oil and cook on a low-medium flame. This will make the potato roast crispier.