Place the refried beans in a small pot with a little water and heat through over low heat. Heat the oil in a large skillet. Add the bell peppers and onions and cook, stirring, 2 minutes. Mix in the cumin, chili powder, salt and pepper and cook 2-3 minutes more, or until just tender. Spoon the mixture into a bowl. Do the same with the refried beans, cheese, salsa, sour cream and lettuce. Place bowls and tortillas on large serving tray. Bring to the dining table and allow diners to make their own fajitas.