The purpose of expository preaching is to discover and accurately explain God's intended meaning in any given passage of Scripture. The grounds for this kind of text-driven preaching is clear when you consider the unique character of the Bible. God's Word is inerrant, and therefore the perfect guide to life and godliness. It is entirely sufficient, so no other source of wisdom is necessary. And it is authoritative, thus we have a responsibility to conform to its principles and commands.
The key to drawing God's meaning out of the text is the literal, historical, grammatical method of interpretation. We read the Scripture in its plain literal sense unless it is obviously using figurative language, we seek to understand what it meant to the original audience, and we thoroughly examine the grammar and syntax to reveal the major features of the author's argument along with the finer exegetical details that often shape and nuance the meaning.
Finally, true biblical exposition explains what the implications were for the original ancient audience and then bridges the gap of time by explaining the implications for the modern hearer. Having fully communicated God's meaning, the expositor then calls all who hear to believe by grace through faith in Jesus Christ, and to obey by walking in humble reliance upon the Spirit of God.