I've been writing hard on the first draft of the Nim sequel - and I always forget what a strange combination of fun, hard work and sheer terror a first draft is. (How do I manage to forget that every time? Maybe because I do love it, so it's best to forget the terror of it once it's done, and just remember the fun of living with these characters.)
Writing a sequel has two big shortcuts - you already know the characters, and you also know the tone of the book. However that also puts limits on the plot, as it has to tie in to the tone of the first book, and be suitable for those characters. There's also some balancing to be done with how much to tell about the first book - not everyone will have read it, and not everyone who's read it will remember the details, but on the other hand the people who've read it and remembered it don't want to be bored by being told everything all over again.
But right now, I'm not worrying too much about those details. I'm just figuring out what happens as they go. As usual, I have a rough plan in my head and know the beginning, the ending and a big scene in the middle. The only problem is that a few nights ago I woke up and thought the scene I was planning for leading up to the end is all wrong for this book, because Nim wasn't really going to be foiling the bad guys herself. I'm getting quite excited about writing the scene the way I see it now, because it's a lot more exciting as well as more Nim. Of course now there'll need to be more changes in the earlier parts of the story.... but that's what second drafts are for – and third, and fourth... and on to the end! Whenever that will be.