It would seem that Jesus makes an appeal to the scribes and Pharisees to be honest in 12:33. He desires these religious leaders to either say He is evil and His works are evil or to admit that He is good, His works are good, and that He is not in league with Satan! To state that "a tree is known by its fruit" is to declare that man is known by his speech and actions. Jesus' speech and actions were good. Their remarks about His power coming from the devil were blasphemous and downright outrageous.
However, although Jesus wants these men to be honest, He verbally admits the hopelessness of attempting to get honest judgment out of dishonest hearts. He plainly informs them of the condition of their hearts; they were filled with poison, like vipers. Their sin was not rooted in their blasphemous speech but in their condition of heart that made such words possible. Their words were evil because their hearts were evil. The same is still true today. Those with good hearts will bring forth goodness in their speech and actions (like Jesus did), and those with evil hearts will bring forth evil in their speech and actions (like the scribes and Pharisees did). It is common sense that what is in the well will come up in the bucket!
"For every idle word men may speak, they will give account of it in the day of judgment" (Matt. 12:36). The word "idle" denotes that which is unfruitful, ineffective, and worthless. This should not be construed to mean that it is a sin to engage in all types of jesting or humor (cf. Prov. 17:22) as well as conversations on secular matters. Jesus' point is that one must be careful with his words--all of them!
Some may have been inclined to think Jesus was too harsh with the scribes and Pharisees for what they uttered, but He implicitly denies that here. Words that are hastily spoken reveal the true state of the heart (e.g., their accusation about His power being from Satan). After all, is it not true that the hypocrite can talk like an angel if he carefully plans to?
The fact that we will be required to give an account of our every word should remind us to heed the wisdom of Solomon found in Ecclesiastes 5:2. Only fools are compelled to say something on every occasion and in every circumstance. There is a time to be silent and a time to speak (Eccl. 3:7).
In Matthew 12:37, Jesus makes "words" the basis for God's judgment. Elsewhere it is said that one's justification or condemnation is based on "works" (cf. II Cor. 5:10; Rom. 2:6) or "faith" (cf. Rom. 3:28). There is no contradiction among these truths. God's final judgment of each individual will be based upon the character of that person. One's faith forms his character, and his words and works are indexes by which the true nature of his character can be determined.