I don't mean to get all philoshophical about this, especially since I don't know jack about such things, but...
We've all had characters 'surprise' us as we are writing a story. And I think the reason for that is because these characters have a certain 'reality' to us. We've built up a profile for our main characters through our past and present experiences with them. We have a definite idea on who those characters are, and thus how'd they'd act and react to various situations.
It's like real life in some aspects. You have an idea as to want you are going to do in the coming day, or how you will deal with some future action based on something previously planned. But until you actually get to that moment in time you don't really know how it will go. As a rule, we are reactive people. We base our actions on the situation and circumstances as they are presented to us.
Similarly, our characters 'react' to the situations we provide in the storyline as they come to them, each according to the personality that we've come to expect from them. Unfortunately, those 'reactions' don't always follow the path that would be most conducive to the original plot of your fic.
Minor side characters can be manipulated to do whatever you want them to because that is their purpose. But as soon as that character becomes 'real' to you, all bets are off.
There are too many examples in my past stories to get into here, but I'll give you one example of a 'reaction' that just happened as it was being written.
In "Love Disabled" Lois confronts the sullen Clark with the challenge; 'If you can look me in the eye and say you don't love me, I'll go'. Clark looks up at her with a defiant look on his face and replies; 'I don't love you.' Then I just watched as Lois took over and walked up to Clark, pulled out her ring that he still wore on the chain around his neck and told him; 'I don't believe you'.
As a writer who generally writes stories to incorporate a bunch of scenes he's imagined, this one wasn't preplanned. It's a great scene, and one that you'd think that you'd have imagined ahead of time. The critical moment of the story 'just happened' because it was what Lois would have done.
Bottom line... let it flow. Most of the time your characters know what they are doing.
Tank (who can think of few stories longer than a vignette that have actually followed the original planned plotline in more than the broadest ways)
