I speak in public quite a bit. Sometimes it is small groups, sometimes it is larger ones. And no matter how much I plan, no matter how I think I know what I am going to say, things change and sometimes take a left. This is why I do not write speeches.
As long as I know what I am talking about, I seem to do better with a small plan instead of a script. Even when I teach classes, questions come up, and in all the years I have been teaching, I still hear questions I have never been asked before. Sometimes I have the answers, sometimes I do not. Other times, I have to wait for the answers to come to me, to be downloaded from the universe (yes, I am strange if you haven't figured that out already).
It is a thing called inspired speaking. You start off with a subject, and start talking on it. Things pop into your head to talk about along the subject line. As you run out of things to say, something else will pop in to keep you talking. This is how I do most of my group or public speaking sessions. I start out, and let my path wind differently depending on what group I am working with and what questions are asked. One talk is never the same as another.
I just works. No plan, no speech, just an idea to pursue. Or as I like to tell people...
I never think about what I am going say. I want to be as shocked as everyone else at what comes out of my mouth.