Cut potatoes in to small 1/4″ cubes. Cook the tilapia fillets in a skillet on Medium heat with the potato cubes. In a bowl, mix melted butter, sour cream, cheese, and soup. Combine the bowl mixture and the tilapia and potatoes together in a 9″ x 13″ casserole dish. Crumble the Ritz Crackers and spread evenly on top of the casserole. Bake in oven at 350 degrees Fahrenheit for 40 minutes. Let stand for 10 minutes after baking before serving. This will serve 8-12 people and goes great with green beans!