"The good person out of the good treasure of his heart produces good, and the evil person out of his evil treasure produces evil, for out of the overflow of the heart his mouth speaks." Luke 6:45
There is, first, a technical aspect to sermon preparation. "What do those words on the biblical page mean?" There are ways of finding out. Not that it's always obvious. But some thoughtful diligence applied to lexicons, grammars and commentaries will usually make the interpretative options clear. Then one uses one's own best judgment.
There is, secondly, a rhetorical aspect to sermon preparation. "How shall I communicate this message in a clear and compelling way? What land mines must I step around -- or step on? How should I locate this biblical truth within the world of the people to whom I am preaching, to help them?" This requires imagination, sympathy, experience, which is more profound than technical exegesis, though the latter is not simple.
There is, thirdly, a personal aspect to sermon preparation. "Out of the overflow of the heart his mouth speaks." That is very profound. Observing preachers through the years, I am convinced this is the secret "genius," so to speak, of great preaching. A man of authentic humaneness, goodness, Jesus-like-ness, might not be a doctoral-level exegete, he might not be rhetorically sophisticated, but that man's preaching will be compelling because he is compelling. Something is flowing out of him, something of Jesus himself. The preacher's good heart, his core being, is well stocked with insights into and personal experiences of the living Christ. He is therefore able to speak out of both the biblical text and his own intuitive knowledge of the Lord into the hearts of the people where they really live. And they are helped, freed, lifted, saved.
My dad was like this. He was a careful observer of the biblical text, but he was no world-class exegete. He paid attention to the obvious in communicating, but he was no rhetorical guru. His preaching had an almost uncanny effect, because his heart was good and the gospel poured out of him. His preaching connected people with the Lord himself.
A boring preacher isn't necessarily a bad man personally, but he might be. A compelling preacher isn't necessarily a good man personally, but he often is. To neglect the personal aspect of sermon preparation dooms a sermon to technical acceptability, with maybe some cutesy rhetorical flourishes. To submit to the personal aspect, to walk with God in total openness to him, putting him first and then preaching by faith, will make the sermon better because the man is better.
That kind of preaching is rare, because that kind of preacher is rare.