Stephen King says he never plans his stories. He says he just creates characters, puts them in problematical situations, and starts writing to see what they do. Stephen King probably lies about other things, too.
There has to be a plan. True seat-of-the-pants writing is fine for NaNoWriMo, where all you care about is word count. But a story needs a plot, and good plots don't happen by lucky accident. Without a plan, your characters will wander and your readers will lose interest.
On the other hand, too much planning can also be a problem. Characters and stories need room to breathe and freedom to surprise us.
I'll be finishing my MFA this summer. My thesis novel, a political thriller, was the product of a 1-semester novel-writing workshop followed by a 1-semester novel-revision workshop. I outlined the plot before I started, then wrote scenes as dictated by the outline.
But I had to make adjustments (or wholesale changes) to the outline every few chapters, because I kept discovering problems (e.g. plot holes) and opportunities (e.g. minor characters stealing scenes and earning larger roles).
I'm still committed to plotting before writing. And I absolutely love the concept of a plotting group. But rather than trying to plan absolutely every turn and twist, I say plan to rewrite and revise. According to many professional novelists, that's the work that takes a book from pretty good to great.