Here’s Dick Lucas’ answer:
“Expository preaching takes the Bible at face value, and that’s why the preacher will want to work hard to understand what it says. The preacher starts with the premise that God must be the perfect communicator, both in content and method. Since he has chosen to communicate through the Bible, we need to work hard at understanding the Bible, so that we can understand what God is saying, because when the Bible faithfully proclaimed, God’s voice is heard. When a preacher goes off on a frolic of his own, his own voice is heard. When someone explains away the Bible, his explanation is heard. But when the Bible is faithfully expounded, God’s voice is heard, because the Bible is the word of God.”
(see in full here).