Yes, topical preaching can be expository preaching. Granted, little of it is, but that does not mean it cannot be. So, I'm asking, "How can topical preaching be expository?" and "When is topical preaching not expository?".
Topical preaching can be expository preaching when . . .
a) the topic itself is derived from the Bible, that is from a passage/passages taken in context. These could be word studies, doctrines, concepts, biographical studies,or activities (praying, evangelizing).
b) the preacher does the hard work of making sure that he has not missed looking at and considering any verses anywhere in the Bible that relate to or inform the topic. Because of this necessity, expository topical preaching is more demanding and rigorous than sequential preaching through Bible books. When preaching through Bible books, the preacher must still relate his passage to the whole Bible (the big context), but the pressure is not quite as great because the passage itself is a safety-check, and he usually has the next week or two to fix something he misses. This suggests the great advantage of sequentially preaching through books of the Bible. But sometimes topical messages are necessary - consider the Sermon on the Mount where Jesus does topical preaching expositionally and in context, or Peter's sermon on Pentecost -same thing. Whether topical or sequential, I would humbly suggest that expositors get in the personal habit of reading the WHOLE BIBLE through regularly, maybe once per month or more. In this way, the expositor is constantly looking for those biblical connections that become part of future messages/series. Does this mean that if the preacher is going to do a series on "love", for example, that he must look at and study in context every usage of this word in the whole Bible? Yes!!
c) it is motivated by a sincere search of the Bible to find the answer/s to a real question or problem that the preacher is asking or has been asked of him. In this case, one might say that the topic initially seemed to come from the culture or circumstance and then proceeded to the Bible for the answers. The preacher must be careful that a) and b) above still apply so that the temptation to give a culturally acceptable answer does not over-ride the responsibility to give a biblically accurate answer (see First Thessalonians 2:3-4 and Galatians 1:10).
Topical preaching is not expository when . . .
a) the preacher is choosing to address the topic because of some preconceived disposition to it. Such preconceptions may include: "that topic is really popular now" (or the latest 'new word' or 'fresh moving' of the Spirit), "that seemed real effective when ______ preached it", "this is my latest hobby-horse or pet peeve", "why don't you ever preach about ________?", "pastors who are preaching this seem to have growing churches", etc.
b) the preacher does not do the hard work of Bible study and exegesis in preparation.
c) the preacher just "borrows" someone else's sermons or topics without doing his own work of Bible study, meditation, and prayer. This is not to imply that we cannot or should not glean ideas or approaches from listening to the sermons of others. I love to hear other good expository preachers!! I have learned from them, and sometimes I import their stuff. But we should not make "their stuff" the main framework of the sermon and then just stick in a few of our thoughts. Our messages must come from the biblical text, and what we glean from others becomes insight or illustration.
d) the preacher "takes a text" and uses it to "springboard" into whatever he wants to say. This kind of looks like the preacher is beginning with a Bible text (promising!) but once he gets started the text is never to be heard from again.
e) the preacher superimposes a personal presupposed "theological motif" (or any other motif) to bend the meaning of a biblical text in a certain direction instead of letting the text speak for itself. A list of examples might include liberation theology (to think of recent news headlines), liberalism, covenant theology, dispensationalism, emergent theology, prosperity theology, denominationalism, Reformed theology, etc. The goal of the expositor is to let the text speak in a manner that exalts the Lord Jesus Christ. The Spirit of God will use the text to show what of a perspective, system, or motif is correct and how.