St. Dorotheos of Gaza: On the true fast

 We need not only to be careful about our food but also to escape from every sin. Thus, as we fast with out stomach, we must fast with our tongue, abandoning slander, lies, idle chatter, reproach, anger and basically any sin that is committed by the tongue. Likewise, we must fast with our eyes so that we do not see idly, nor do we act boldly with the eyes looking impudently. Also, we must prevent our hands and feet from performing any evil.

This fasting, as St. Basil says, is an acceptable fast, as we abstain from every evil manifested through all the senses. We proceed to the Holy day of Resurrection, as we have already said, renewed, clean and worthy of the communion of the Holy Mysteries. 

+St. Dorotheos of Gaza, from Abba Dorotheos Practical Teaching on the Christian Life, lesson 15 On the Holy Fast.

St. Anthimus the Georgian: The true fast is that from sin

 What profit is it for the body to be empty of food, but the soul full of sins? What profit is it to be pale and withered from fasting, but burning with envy and covetousness? What profit is it to abstain from wine, but to be drunk with the venom of anger? What profit is it to not eat meat, but to rend the flesh of our brethren by slander? What profit is it for us to cease from things which are sometimes allowed, but to do things that are never allowed? God loves and honors those who keep themselves from what is forbidden.
+St. Anthimus the Georgian, from Romanian Patericon 147, Works and Teachings #29.

St. John Chrysostom: What true fasting really is.

Do you see, dearly beloved, what true fasting really is? Let us perform this kind, and not entertain the facile notion held by many that the essence of fasting lied in going without food till evening. This is not the end in view, but that we should demonstrate, along with abstinence from good, abstinence from whatever is harmful, and should give close attention to spiritual duties. The person fasting ought be reserved, peaceful, meek, humble, indifferent to the esteem of this world. You see, just as one has neglected the soul, so it is necessary to neglect empty esteem as well, and to have regard only for Him who examines our innermost being (cf Ps 7:9) and with great care to direct prayers and confessions to God, and provide for oneself according to one's ability the help that comes from almsgiving.
+St. John Chrysostom, from Homilies on Genesis, homily 8 #15.