*finally remembers to post on thread*
I'm an outliner, so I have this problem occasionally. Lately, it's not so much that my plot is expanding. In some cases, I'm not even adding a lot of scenes. Rather, my scenes are turning out a lot longer than I expected. They're going into greater detail, or else the style is more rambling, or there's two pages of character internal monologue before the action starts.
I don't think this is a "problem" as long as the extra words help tell a good story. If the words are warranted or necessary, they are not wasted. However, it can be extremely annoying to have a novel turn out significantly longer than you expected because it can upset your goals and long-term plans.
I had this issue with my NaNo novel. The style was a lot more verbose than expected, so instead of finishing my novel during NaNo, I only got about a half or two-thirds done. Peter's Angel just threw me for a large loop in this regard... My scenes are turning out significantly longer than I expected, plus I have more scenes than I estimated. Instead of 120,000 words, my book is threatening to be somewhere in the 200,000 range. To accommodate I have to set back my goal for finishing the novel by at least a month. I'll survive, of course, but it's a little disgruntling!
