1Brush grill rack with vegetable oil or spray with cooking spray. Prepare the coals or a gas grill for direct heat. Heat to medium heat, which will take about 40 minutes for charcoal or about 10 minutes for a gas grill.
2Cut fish into 4 serving pieces. Brush fish with about half of the butter; sprinkle with the salt and pepper.
3Place the fish on the grill rack (if fish fillets have skin, place skins side down). Cover the grill; cook 10 to 14 minutes, brushing 2 or 3 times with remaining butter, until fish flakes easily with a fork. Serve with lemon wedges.