St. John Chrysostom:
Fasting is the change of every part of our life, because the sacrifice of the fast is not the abstinence but the distancing from sins. Therefore, whoever limits the fast to the deprivation of food, he is the one who, in reality, abhors and ridicules the fast. Are you fasting? Show me your fast with your works. Which works? If you see someone who is poor, show him mercy. If you see an enemy, reconcile with him. If you see a friend who is becoming successful, do not be jealous of him! If you see a beautiful woman on the street, pass her by.
In other words, not only should the mouth fast, but the eyes and the legs and the arms and all the other parts of the body should fast as well. Let the hands fast, remaining clean from stealing and greediness. Let the legs fast, avoiding roads which lead to sinful sights. Let the eyes fast by not fixing themselves on beautiful faces and by not observing the beauty of others. You are not eating meat, are you? You should not eat debauchery with your eyes as well. Let your hearing also fast. The fast of hearing is not to accept bad talk against others and sly defamations.
Let the mouth fast from disgraceful and abusive words, because, what gain is there when, on the one hand we avoid eating chicken and fish and, on the other, we chew-up and consume our brothers? He who condemns and blasphemes is as if he has eaten brotherly meat, as if he has bitten into the flesh of his fellow man. It is because of this that Paul frightened us, saying:
“If you chew up and consume one another be careful that you do not annihilate yourselves.”