I'm not a fan of writing detailed plot outlines, but I do like to have a list of scenes ready to go when I write. I like to know the ending and approximately how much material I will need to complete the book. I'm currently working on Plague, the second book in the Dead Land Trilogy. As the action is unfolding, I'm finding one of the main characters separated from the group. I didn't plan for this to happen, but I like the possibilities. Can he survive on his own? Will he be reunited with the group? How will it happen?
Don't ever be afraid to take a detour from your outline. It can take your story in new and exciting directions. And if you're surprised and delighted at the turn of events, it's a good bet the reader will be, too.