Add the rice to the melted butter or margarine in a large skillet and brown over medium heat, stirring. Add soup and water, cover, and cook for about 10 minutes. Add green pepper and celery and continue cooking, covered, for about 15 minutes longer or until the rice is tender. Add turkey and mushrooms (we use the stems and pieces) and mix well. Pour into a greased casserole and bake at 350 degrees F for about 20 minutes. Top with cheese, return to the oven, and bake until the cheese is melted.