What should my reaction be if someone commits disbelief (kufr) in front of me?

Falling into disbelief (kufr) is one of the most reprehensible evils, and it must be denounced according to one's capacity, provided that it does not lead to a greater harm. Thus, whoever can denounce it with proof and clear explanation, so that the disbeliever may return to Islam, must do so. As for the one in authority, he should compel the disbeliever to repent, and if not, execute him, provided the conditions for declaring someone a disbeliever are met. As for those not in authority, they are only required to denounce it verbally. And whoever is certain of a person's disbelief must treat him as an apostate. So, his wife should not allow him conjugal relations and should strive to free herself from him. And whoever is sought by him for marriage should not give him his female ward in marriage. And whoever finds him leading prayers should not pray behind him.