I would strongly encourage preachers to run through their message, out loud, before preaching it. Call it a rehearsal if you like, or use a different term if that makes preaching sound too much like a performance. Preaching is primarily an oral and aural art form, so to restrict preparation to written and read forms makes little sense. A good outline should reflect the content and structure of the message. A good manuscript will reflect this plus the details and word choices. But running through the message verbally will help to clarify the flow, the efficacy of transitions, and so on.

New preachers tend to speed up. If you haven’t preached much, there is a good likelihood that your actual sermon will go quicker than the rehearsal. The story is told of Billy Graham, preaching for the first time and getting through his message so fast that he ended up preaching four messages in something like 10 minutes. (If you have a reference for the true story, please share it below.) Strong nerves tend to speed up delivery. Knowing this may help you to slow down delivery and time your practice appropriately.

Experienced preachers tend to take longer. Once the nervous reaction of putting the gas pedal to the floor and speeding through the delivery is overcome, the situation changes. Preaching through a message to an empty room is one thing. Preaching to dozens of faces, with all the feedback and distractions, is another. Unless you are a preacher with your head permanently stuck in your notes (search on “no notes” above!), you will subconsciously sense facial and fidgety feedback from your listeners. This will lead you to expand, restate, illustrate more, and so on as you go through the message. The result is usually that you need longer for delivery than you did in practice. If you need to preach 45 minutes, then practice aiming for 40. If the goal is 30 minutes, then practice towards 25-27.