I have to agree, KnightShade. I've found, to my detriment, that forcing a character to act outside of their chosen personality is a good way to lose hold of the character. I suppose some writers work within the plot and fit their characters to the story line. When working alone, I can do this. But then I go and do some silly thing like... offer a story idea to a bunch of strangers and they go and muck everything up! Next thing I know, my plot is more of a guideline and my main character is going belly up because he really doesn't like the girl he's supposed to and his horse has an attitude problem because of some imp up his butt. Then the storm hits and everything goes to hell in a handbasket.
At that point of time, I can only hold on and "feel" my way through it via the character. Is he the kind to leave the horse out to drown in an attempt to get rid of the imp, despite the fact that the horse is a pivotal part of the blood letting he'll need for the magic amulet later, or will he drag the beast inside his tent and get sat upon and break his leg? Either way, the story is gone and we're in uncharted territory.
Then again, that might be just exactly why RP is so fun.