One-on-one (and sometimes while teaching,) I try to check-in, because I find a lot of the time my audience will know stuff about my subject that I didn't learn until I went to grad school for it; but then I find that people who learned, say, "Evolution" in school have what I consider gaping holes in their understanding and knowledge.

It forces one to be more interactive than discoursive

When giving talks, there's a knack to laying out the groundwork of a subject in a way that's explanatory to the unfamiliar, while feeling like a nice review of the issues at stake to the familiar. I haven't quite mastered it, but it's really impressive to see it in action.