Be sure to use margarine instead of butter, because margarine is oilier and seems to conduct more heat. The extra heat, plus the additional oil, develops the gumbo fil to a more desirable taste, texture and color.
Some people prefer to skim the oil off before serving.
If the gumbo is made in advance, do not add the seafood. When ready to serve, bring the gumbo to a rapid boil, lower the heat to a simmer and add the seafood. Immediately cover the pot, turn off the heat, and let the pot stand covered 6 to 10 minutes. In a 4-quart heavy soup pot, melt the margarine over medium heat. Add the onions, celery and bell peppers.