The efficacy of a preacher’s words to produce biblical fruits and conversion is directly related to the message’s faithfulness to the Scriptures.
Eloquence (1 Corinthians 2:1), enticing words full of man’s wisdom (1 Corinthians 2:4, 13), and fleshly wisdom (2 Corinthians 1:12) bring nothing to the table. The men God chose in the Scriptures were slow of tongue and speech (Exodus 4:10) or so long-winded that people fall asleep and fall to their deaths (Acts 20:9). Their messages both were efficacious and have affected millions (if not billions) throughout history. This is because their messages were biblical.
It is a curious thing that eloquent, out-going, charismatic preachers so easily attract the large crowds but make such a small impact on anything but television ratings and book sales figures.
My friends, if you must preach but are afraid because you don’t feel like you have the personality for it, preach. If your message is pure, God will bless it as He has always done. And even if your delivery is so long-winded or boring that people fall asleep and perish, well, it wouldn’t be the first time!