Actions Do Speak Louder than Words
This is an obvious statement, but it is no less true in fiction than it is in real life. You shouldn’t want your characters to preach any more than your narrator. We get plenty of the do-as-I-say-and-not-as-I-do-crowd in real life. Why would we expect readers to want to deal with that kind of hypocrisy in a book? If you have created a character who believes strongly in something, then let them act on it even before they talk about it. A character who speaks out about animal cruelty in a story should definitely have pet of some kind. Let your character who is pro-life or adamantly against child abuse volunteer at a place that provides pregnancy support services or as a Court Appointed Special Advocate (CASA). Christian characters should behave in a Christian manner toward their neighbors—and if they have to work really hard to be nice to a nasty old man who lives next door, so much the better, because your readers will be able to relate to such a challenging but probable scenario.
Of course, if you have an antagonist who is thwarting your protagonist at every turn, let him be the hypocrite, by all means. It will give your readers even more reason to support your protagonist. It will also provide a nice contrast to your protagonist, who is practicing what she preaches.
Present Both Sides for Realism
If you really want to make a point, be certain there is an antagonist and that he is as reasonable-sounding as possible. After all, not all of your readers will be on the same side of your issue as you are, and your point will be that much more effective, if your antagonist appears just as reasonable as your protagonist—even though he is wrong, of course. And the more emotionally-charged the issue you’re confronting, the more important it is to have your protagonist behaving and speaking in a logical, well-informed, rational way. But be sure and do her research for her, so she gets the facts right when the confrontation with her antagonist occurs!
The good news is that in fiction, you control the scene and how all your characters respond to what’s happening. You can have a violent confrontation or a quiet, one-on-one meeting or any combination thereof. It’s completely up to you! That’s the beauty of being the writer! Just remember: Let your protagonist do all the talking. Let her words and actions make your point for you.