Even before I do any character mapping or world building, everything becomes real. Some characters aren't even fully fleshed out but they seem as real the characters I've spent a lot of time developing. Their situations fit so well into the previous novel without me planning it that way. Even the world, which undoubtedly needs some work, becomes a real place in my mind.
Although I've worked on 2 novels extensively, they're still pretty rough. When I first wrote them, I didn't intend them to be a part of a series so, now I have to take into account other books will build off of them. The novels I plan on working on well into the future, I write in a journal only. To me, putting it in a Word Doc is a commitment to seriously work on it and right now, those novels are really really rough drafts. I call them the draft before the draft.
Despite this, I've been able to keep to my one-novel-at-a-time plan. I'm not feeling overwhelmed and I don't feel I'm neglecting the novel I'd been working on for months simply because another came to mind. Wiring the new novels only in a journal helps me prioritize. All my novels are important but the ones on the computer get my attention first.
I wrote the first novel of the first series years ago and loved it. Now, I'm going through it again and realizing some of the mistakes I've made. I've learned and am still learning about writing. The second novel in the series is one I've been working on for a really long time, like since I was a kid. I actually started working on this one well before the one I mentioned above. The idea stayed with me and kept developing. I've rewritten this story so many times but kept the same characters and same basic premise. Maybe it's not the story itself but the characters that continue to grow, resisting any effort to be shelved.