Good faith is a principle that guides all international law. The Vienna Convention has incorporated it for every moment of the life of the treaty. The rule of interpretation contained in Article 31 binds every interpreter. In terms of international jurisdiction in the field of Human Rights, this rule requires the judge to consider the particularities of the societies bound by the treaties. In this case, good faith is not a mere reference to a fundamental principle of International Law. It obligates the judge to a conduct that is the basis or assumption in the act of applying the rule of said article. The paper will refer itself to a possible absence of good faith in the application of those methods by the International Court of Human Rights, resulting in judicial activism. In this case, the presentation will refer to the Lagos vs. Peru judgment.