A lot of recipes call for a great deal of oil or other fat when cooking. Most dishes can get by with less oil and roasting potatoes is a good example. Because fats won't evaporate like other liquids, they doesn't have to be replenished during the cooking process and a little bit goes a long way.
Another approach is to use a spray oil (see Oil Spray). Simply spray the bottom of the roasting pan lightly and then coat the potatoes (or other vegetable / fruit to be roasted) with a light spray of oil.