There is an age-old feud amongst writers: Plotters versus Pantsters. That is, should you plot out the course of your novel, or should you fly through it by the seat of your pants?
In my opinion, you should do both.
I've been on both sides of the feud. I wrote my first novel completely by the seat of my pants, blindfolded and with no map at all. There was something wonderfully spontaneous about it, like I was living on the edge all the time. Every time I sat down to write I was given a new surprise, a new twist that took my breath away. I loved the freefall of it.
But the novel took me five years to finish, and then another year of extensive edits to get it in shape to submit. Granted, that draft got me my agent. But the novel didn't sell, and a big reason, I believe, is because it still needs editing, and a lot of the stuff that probably needs to go is stuff I found while I was on my wild ride and refused to let go of.
When I started my second novel, I didn't want to lose that spontaneity, but I didn't want to take six years to write it. So I sketched out a loose outline using The Hero's Journey (a book by Chris Vogler - highly recommended) template. Then halfway through writing the book I realized that the story needed to start a lot later so I started rewriting the whole thing. I never adjusted my outline, and so once again I was flying virtually by the seat of my pants. That book remains unfinished.
With my third novel, my current WIP (work-in-progress), I decided that I wanted to finish it in less than a year. That meant I needed a strong outline, a clear road map of where I needed to go. I took a class in Plotting Via Motivation (with WriterUniv.com - also highly recommended), I drudged up all my old notes from various plotting workshops I'd attended at conferences and I dragged out The Hero's Journey. I put together a hero's journey template for my MC (main character) and even worked out a scene plot. I'm on track to finish the book within the time frame I've given myself.
The important thing is that even though I have this road map, I'm allowed to veer off it. That outline isn't chiseled in stone. Flights of fancy are of course allowed because characters aren't always predictable and sometimes things happen that surprise us. For instance, I just had a character pop up that I had not planned for or even heard of. But there he is. So I've got to adjust my scene to allow for him. There's still spontaneity. But I have found that spontaneity within a structure is ultimately much more productive - and time-saving.
Writers will debate plotting until they're blue in the face. Terry Brooks swears by outlining. Diana Gabaldon doesn't. Both are fantastic writers. You can't tell from the finished product whether a writer used an outline or didn't. (Well, sometimes you can, but that's a different blog).
It's all about which process works best for YOU. And it may even be dependent on the story. Maybe my first book needed to be written in that meandering, fly-by-night way. My current WIP needs structure. The important thing is to listen to yourself, to your characters, and decide.