William Taylor in his 1876 work The Ministry of the Word lists three qualities of the preacher of an effective sermon, earnestness, courage & tenderness.

Earnestness springs from two sources: a personal conviction of the truth proclaimed and the certainty that those hearing need to know this truth.

But before he gets into earnestness and its causes, Taylor begins by stating what earnestness is not:

We must not confound it with mere vehemence of manner

We must not confound it with mere vehemence of manner. Rant is not intensity, neither is noise earnestness. Too often the ” sound and fury” signifying nothing;” and sometimes as I have been compelled to listen to preachers of the noisy school, I have thought that they had taken their cue from Quince in his description of the lion’s part, when he says,  “You may do it extempore, for it is nothing but roaring.”‘- That is, and always must be, ridiculous …

I have often seen men who thought that play acting with lots of noise, as if pretending to enthusiasm were a good substitute for being earnest.

Taylor explains that one can never be earnest unless one is first convinced of the truth of the matter:

If we have not made up our minds upon a subject, we cannot kindle into enthusiasm over its treatment ; and he who has not yet brought the ends of his thoughts together on any matter, should keep that matter out of the pulpit until he has. It is the irrepressible in a man that makes him earnest. If he can keep anything in, then let him keep it, for such a thing, generally speaking, is not worth letting out, and his utterance of it will have no force.

Unfortunately, too often, the preacher goes into the pulpit without conviction, and so he merely talks:

The preacher fills up the time with talk, because he must say something. He does not go into the sacred desk under the absorbing impulse of the feeling that he has something which he must say. So he is aimless and uninteresting, and fails to impress others because he is unimpressed himself. It cannot be too constantly remembered by you, that your usefulness to others must depend, next to the influence of God’s Spirit, upon the intensity of your own convictions. There is nothing so contagious as conviction.

As Taylor says, “If, therefore, you have no positive convictions, keep out of the pulpit until you get them.”

Now, the preacher must not only be earnest in his personal conviction of the truth, he must earnestly believe that others must hear this truth. The preacher must be convicted of the truth and of the importance of that truth:

But another element of earnestness is a vivid realization of the position of our hearers. Let a man have the firm belief that he is dealing with immortal souls ; that unless these souls embrace the Lord Jesus Christ and live in obedience to His laws, they must perish everlastingly ; and that he is set to persuade them to choose that ”’ good part which cannot be taken away ” from them, and he cannot help being earnest in his appeals to them.

And again:

There is no mistaking the earnestness of him who runs from the burning dwelling to cry ” Fire! fire!” He sees the evil; he knows that if means be not taken promptly to extinguish the flames the house must be destroyed ; and so he does not take it leisurely, but rushes on along the nearest way to the engine-house. And it is the same in the pulpit.

In conclusion:

Here, then, are the twin sources of that earnestness of which so much is said, namely, intellectual conviction of the truth of those things which we proclaim; and loving realization of the fact that our hearers need to have these things said to them in order to be saved from the evils of time and the perdition of eternity. Give us these in all the occupants of all our pulpits, and the world will be constrained to listen to them. There is no royal road to earnestness; neither can it be successfully counterfeited by any histrionic art. We can gain it only through personal conviction and pervasive love ; but, when we do gain it, we do not so much possess it as it possesses us, and carries us out of ourselves to achievements which are as astonishing to ourselves as they are irresistible to those whom we address.

The discussion of earnestness is found in Lecture VI, pp. 131-138.