I'm currently in the process of writing chapter 8 of Rosetta, the chapter which will introduce Eli, a 24-year-old werewolf who will become quite important throughout the rest of the book and future ones. Eli was probably the second or third character I created, and the first one I had actual expectations for. I know what I want him to do by the end of the book, and while a whole host of other things need to fall into place to make it work, the thing that frightens me most is the Eli won't be the character I need him to be to make those things happen.
This happened to me once before. I started a novel with the intention of the main character getting together with this one guy. And when I got to the part of the outline where they were supposed to get together... nothing. No sparks, no chemistry, no conceivable reason why these two people would ever see each other in a romantic light. He was her sons' principal; she was his students' mother. Nothing more, nothing less.
Instead of rolling with it and changing the plan, I panicked and shut down. This was the concept I'd built my whole story around. If it wasn't going to work, what point was there in continuing it?
I'd like to think I've grown from this, and that I can readjust if Eli is not ready to do what I need him to do at the end of the book. I'd like to think I'm a better writer now than I was when I was 18. And I'd like to think I'm more flexible now. But the problem with having expectations for a character is that then they have the potential to disappoint you.