What eveyone has been saying is that when using forzen veggies, they have already been par-cooked and just need to be warmed. This is quickly done by tossing into boiling water, steaming, and micro-waving. When done by heating in a pan, expcially and uncovered pan, it will take longer to heat them up.
If using fresh veggies, then saute, steam, broil, or microwave until the veggies are hot, and lightly softened, with a good crunch left in them. If you saute, the flavor of the oil or butter will flavor the veggies. Steaming, boiling, or nuking won't add any flavor. Also, the the veggies will caramelize when saute'd or broiled.
So, by using different heating methods, you can really taylor the recipe to your tastes. Steaming and microwaving will give the veggies a "brighter" flavor, while the other methods will alter the natural veggie flavor.
Seeeeeeya; Goodweed of the North