I “pantsed” the first few chapters of Steelheart, but I quickly went from there to creating an outline. The early part was exploration, the first three or four chapters. That’s not uncommon, even for an outliner. However, I did then stop and produce a really solid outline for the book. (Actually one of the most solid I’ve ever made.)
When you’re discovery writing, you often have a lot more success creating and discovering characters, in my experience. That’s why I often free-write a few opening chapters to a book, so I can get a feel for who these people might be. However, a difficulty with discovery writing (writing by the seat of your pants) is plotting—it’s very difficult to create a tight narrative without an outline. (That said, many people who love to discovery write can fix this problem in revisions.)