Your cream soup should be thick, not runny, when you add it to your green beans for making green bean casserole. There is usually too much liquid if your green bean casserole is runny. This can happen if the vegetables are not cooked long enough on the stove, or the cream soup is too thin. To fix a runny green bean casserole, stir in some corn starch and place it back into the oven for another 10 minutes, until the casserole is thicker and the center is bubbly.