"Their conversation took a drastic turn"

My girlfriend and I had a good laugh last night when I explained how a part of the story I'm writing didn't follow the outline.  I was explaining to her that certain events were supposed to happen, but because of a conversation between the two characters things changed.  She didn't really get how fictitious people that I created in my head could somehow pave their own futures.  In the end what happens to them is up to me, but that doesn't mean that their personalities and actions don't cause 'rifts' in the story.  Just one of the many things that make being a writer so exciting.  To somebody that doesn't write I can see how this sounds like I may need to get professional help for the voices in my head, but hey, fuck em.  I'm already proud of the characters and their lives and I'm still in the beginning.  I'm the one writing it, but am just as excited as a reader to see what comes next. . .