I think that's one reason I don't plot many of my stories out in advance - I just have to keep good notes on what HAS happened so I can have it make sense later.
I've never plotted things out either. However, I do have 'waypoints'. Specific tasks the story has to meet, however where they occur and what happens in between is completely up to the characters. As long as I reach the ending, and the entire story hasn't lost it's focus, I figure I'm on target.
A 'minor character' is simply one who doesn't have a major role. You have the primary male and female lead (protagonist and antagonist, though they aren't the same). Main characters are those where the story wouldn't work without them (i.e. they couldn't be yanked without the story not working).
Secondary characters are characters who appear often, but who aren't 'essential' to the story, whereas minor characters are the multitude of 'non-essential' characters, where you could easily substitute virtually anyone else in their role (i.e. they could either be a 64-year-old man or a 17-year-old snot-nosed kid, their roles are simply there for the taking.
Anyone have any better definitions?