What is a sermon? Well, it’s a whole life brought to bear on a text each week for a single 30 minute or so sermon before a specific congregation. It shames the preacher not to be who he or she is in the pulpit, and to pretend to be someone else. It de-localizes the sermon from the local context. It distorts who the preacher is before the congregation.
This is why Philips Brooks’ famous quote is so true: “Preaching is truth poured through personality.”
This is also why some guys don’t last long in ministry or last long but only because they short-time several pastorates: How effective you are in the pulpit is directly tied to who you are out of the pulpit. That’s why the discipline of holiness and the patient and gentle care of the people God has placed under your care is so important.