1. We are dealing with two “authors” when we preach from the Gospels. We have Jesus telling the story to a specific audience in about AD30. Then we have the inspired account from Luke or Matthew, etc., some decades later, potentially to a very different audience, and most likely in a different language! The focus of the inspired writer is on the authorial intent of Jesus, so rightly we focus there. But we must see that the writers were inspired to weave together these narratives so that in their arrangement there is meaning conveyed. We need to keep both authors in view.
2. Sometimes we are dealing with more than one account of the same parable or life event. If we don’t compare the accounts we may preach our specific text with inaccurate detail. For instance, caught up in the presentation of the feeding of the 5000 we might get carried away with their plight and describe the terrain as arid or dry (and then have some avid listener point out that the grass they sat on was green from Mark’s rendition). This detail in Mark is not incidental. It fits with the emphasis Mark is conveying, but is irrelevant to the other gospel writers. Be sure to check the others for accuracy.
3. The different accounts offer us more than accurate harmonization. Checking two accounts will allow us to be more accurate in our telling of the story. But more than that, careful comparison will enable us to spot the emphasis in our specific text. What did our specific Gospel writer want to convey? The details included and omitted will help us to determine this (as well as context, flow of narratives, etc.)
4. The different accounts may tempt us to preach the harmonization. Generally I don’t think this is a good idea. Our goal is not to make a composite sketch from apparently inadequate eye-witnesses in order to try and come close to the reality of the event itself (I do not believe they were inadequate at all). Our goal is to faithfully preach the inspired text of a specific writer. There is value in harmonizing, but the goal is to preach the text, for that is what is inspired.
Gospel stories, both life events and parables, can offer challenges to the preacher. But they are so wonderful, I hope I don’t even need to encourage you to preach them, and to preach them as well as you can.