The good faith standard, in itself, does not alter, or extend, the terms of the contract – it merely prohibits conduct that frustrates the explicit agreement:
Illinois like other states requires, as a matter of common law, that each party to a contract act with good faith, and some Illinois cases say that the test for good faith “seems to center on a determination of commercial reasonability.” The equation, tentative though it is (“seems to center on”) makes it sound as if, contrary to our earlier suggestion, the judges have carte blanche to declare contractual provisions negotiated by competent adults unreasonable and refuse to enforce them. We understand the duty of good faith in contract law differently. There is no blanket duty in good faith; nor is reasonableness the test of good faith.
Thus, good faith merely polices subjective bad conduct in the performance of the contact. It is in this sense that good faith in contract is intention-implementing. It is in this sense that Professor Burton reports: “In my view, courts generally do not use the good faith performance doctrine to override the agreement of the parties. Rather, the good faith performance doctrine is used to effectuate the intentions of the parties, or to protect reasonable expectations, through interpretation and implication.”