Pastors sin in this area more often than you might imagine. Some of them waste a lot of time during the week–––occupied not with godless affairs, to be sure, yet not with the one thing needful. Then Sunday rolls around, and there they are, standing in their pulpits, unprepared to give their people the best that is in them. Their listeners get the impression that the pastor is merely reciting something because he has to, regardless of whether the sermon actually helps anyone. That is a horrible situation! …
But if a preacher talks without a plan and purpose, he need not wonder that he is not achieving his goal. Shame on pastors and students preparing for the ministry who go to work in a sloppy and careless manner, jotting down and reciting anything that comes to mind, anything that flows into their pen and anything that leaps from their lips! As a rule, that is what happens when a pastor preaches off the cuff. What I mean here is not only pastors who have plagiarized their entire sermon but also those who have not adequately meditated upon the subject they intend to present to their listeners. But after thoroughly meditating on it, their flow of words is much better.
Law & Gospel: How to Read and Apply the Bible by C. F. W. Walther (A Reader’s Edition)