One of the differences I've very recently realized between writng nonfiction and fiction relates to their very nature. When writing a fiction, one of the cardinal rules is that as an author, I must "show" instead of "tell." This is the wrong way:

Sharon was angry.

That's "telling."

This is the right way:

Sharon growled, yanked the dishes from the dishwasher, and slapped them down one on top of the other. Then she she kicked the dishwasher closed with one foot, making the plates rattle.

That's showing.

Now, in nonfiction, if one is relating an incident, then the tools of fiction, the showing rather than telling come into play. But if one is doing exposition, explaining something, giving a sermon as it were, then one must explictly tell. Showing isn't enough. One must connect the dots, and do it carefully and clearly. And then you must tell your audience what your point is, not expecting, as with fiction, that they will puzzle out that Sharon is angry.