This is one of the most neglected areas of homiletics. I admit I struggle with it myself. The sermon stays with the mood of the biblical text. This is a frequently (should i say, almost always) neglected part of preaching. Sometimes, the personality of the preacher including his humor can take over the biblical text. As a result, the audience will feel the rhetorical force of the preacher rather than the bilabial text. A sad text becomes a hilarious text. A humorous text becomes a condemning text and so on.
For expositional sermons to be expositional, the preacher is not only responsible for the author’s meaning but also the sense. If possible, the preacher must understand the author’s rhetoric and goal. If the matter is quite grave, it is probably inappropriate to set the tone of the sermon to be humorous. The audience may like the jokes, but they will once again be lost from the message. The best question for preachers to ask themselves before going on stage is, “HOW would the biblical author deliver this message?” The answer reveals a lot about the preacher’s understanding of the text. Stay with the mood.