And then a character decides to do whatever they want and the whole scheme is thrown off.
Sometimes, it's inconvenient. I try to push them toward where I want them to go, but there's heavy resistance and gnashing of teeth and fighting--sometimes there's growling.
I find its often easier just to let them do that. Sometimes, it's great. It leads to wonderful character development. They can discover themselves, do their own thing. Going off tangent though, it can be troublesome. How will I finish my envisioned masterpiece?
The answer? I won't. Instead, I make a new masterpiece. I improvise. I go with the flow and let my characters go wild. I let them become real. And that's really what it boils down to: characterization. To have real, believable characters, you have to let them be real characters. Let them develop on their own. Let them grow. Let them cook up plots and have their own ideas.
And who knows? Something good might come of it.