Making it up as I go along

I’ve always been fascinated by how someone can attempt a book series. Given how much I rewrite and rework between drafts, I’ve always worried that if I tried a fantasy trilogy, by the time I got to book three I’d discover that everything I’d written in book one doesn’t work when the time comes for the big finish.
One of the things that intimidates me about the Applied Science series in Big Pulp is that what I’m doing now will change the book. I’ve decided Claire, a supporting character, has been friends with one of my leads, Danielle, for several years before Brain From Outer Space starts. That means she also knows Dani’s boyfriend Steve, and that when she hits on him in the book, it’s going to have a whole different feel than when it was wright after they met and she didn’t know Dani at all.
Of course, Dani and Steve’s relationship will be different because I’ll have explored it through several more stories and I can feel it changing in my head. All this is … intimidating. Especially since I’ll need to work on finishing Brain well before I’ve finished the story series.
On the plus side, I was reading the first volume of Fantagraphic’s Complete Peanuts, and I was struck by Charles Schulz’ statement in an interview (included in the book) that when he started a story sequence, he didn’t plan it out for fear of having to resort to weak gags to keep the plot going. Instead, he concentrated on the characters and let it unfold.
And it certainly turned out alright for him, didn’t it?