In my completed manuscript, which is now on its fourth edit and will be ready for agent representation search somewhere between early 2018 and spring 2018, I thought I'd share an interesting (to me at least) twist in the earliest stages of my process. Before I ever started writing I had a story idea and I spent four or five months researching, collecting interesting tidbits and primarily just thinking about the story. A month or two in I decided to add a character that would complement the story. The more I thought about the book and the story arc, the more I found myself thinking about this new character. The new character was much, much more interesting to me than the other characters in the story. Eventually this new character was almost all I thought about. Then another month in I came up with two characters that would complement the new character I liked so much. These two new characters were also more interesting to me than the original characters I had developed at the earliest stages of my thinking. Eventually I decided that the first new character would be the main character and then the two add-on characters would be the primary supporting actors. Once I decided that I added yet another new character who would be their supervisor/leader, but basically a complementary piece to the story. The characters I originally thought would be the stars of the book ended up being only bit players with one exception who played a decent-sized supporting role. By spending many months just thinking and not writing, my characters and story line underwent a major evolution. Just thought I'd share.
I'm still doing this even in the writing stage When I was writing my Doctor Who fanfic, I came up with a character after having written about 4 chapters, so I re-wrote chapter 1 to introduce her. By the end of the story, she'd gotten more of the word-count by POV than any of the characters I'd originally started the story with As opposed to my Urban Fantasy WIP, which went the opposite direction: I originally imagined that my cast of main characters would be Alec: first-person narrator Charlie: leader of her group Amy: Alec and Charlie's friend Jason: Amy's brother Then I realized that Amy would be spending most of the book unconscious in the hospital with Jason at her bedside, so the breakdown suddenly became Primary protagonists: Alec and Charlie Secondary protagonists: Amy and Jason Except that Alec was such a "Charlie's The Boss, do as she says" follower-type, with his main contribution being to follow Charlie's lead, that Charlie turned into the singular main character (with Alec as her First-Person Peripheral Narrator) as opposed to just the first among four equal main characters.
Oh yes. I do the same thing, and had the same kind of result with my novel. I think a large portion of 'writing' is done away from the computer or any writing tools—other than a small (bedside or pocket) notebook to scribble ideas into. Thinking and envisioning is not time wasted at all.
I didn't do this with my MC, but his girlfriend got replaced by another girlfriend later in the story because he liked her better (actually I liked her better too, but that's our secret). But my MC is a good young man who doesn't mess around on his lover, so I had to rewrite the story to have girlfriend one dump him, and girlfriend two saves the day and his heart. And that's how I accidentally pantsed my way into romance.