I might have an insight into this.
In my paying writing, the plots are very pre-determined, and the thumbnails of the characters likewise. What I have found here is that as characters get fleshed out in the writing process, they will develop quirks and so on to round them out from: "He; early 30s, very driven to succeed at work, private life takes second place to this. Trying to clear up mess brother made of family firm, and resenting it. Has focused to the extent he's lost sight of having a life."
These quirks are what bring the character to life, and they're more or less inevitable given that the character needs something to break them out of the straitjacket of the pitch premise.
In my hobby writing, I'm a pantser. Development of characters is more unpredictable. I don't know exactly how they'll turn out. Disappointingly, I find that where inspiration doesn't strike and the character is still struggling to emerge, I tend to fall into my familiar patterns.
David, that is very interesting, thank you for sharing. I have always evolved my work and while sales have not been phenomenal, they have been decent, especially back in the mid-2010s, so I did not feel a need to change my approach. On one hand I do like the unpredictability of characters 'appearing' but then do worry if the characters are then simply 'plot devices'. Interestingly reading criticism of Season 8 of 'Game of Thrones' I saw people complaining that in contrast to the books, it was 'too character driven' rather than plot driven.
Writing is my prime hobby as well as a money-making activity and I like the fact that it does not feel the way writing reports or resources for my full-time job does, which maybe why I like spontaneity in where my books go as I get lots of 'highly plotted' writing in my day-to-day work with input far more critical than any editor of novels would give.
After reading this article, I did consider whether there were any characters who particular 'grew' as I wrote and in fact though I evolve my writing, in most cases I could think of - and I must say I cannot recall all the characters from my short story anthologies of the late 2010s - there was always going to be someone in their role, but perhaps not exactly the way they turned out. In
Eve of the Globe's War which was filled with lots of parallel characters from our reality, Écuyesse Servane de Grimoard grew from being a minor character who was there primarily to show how lazy the MC was in challenging injustice to being a heroine and ultimately a love interest. Perhaps the character ironically that grew to be really significant in a story was in one of my non-AH novels,
Folly. The Danish woman Hedvig Schmidt was meant just another of the MC's friends but ultimately her energy meant she effectively became the leading detective of the murder case and her role grew massively beyond what was originally envisaged.
These two examples lead me to think that while with evolving novels, characters can appear as simply tools for advancing the plot, if they develop strongly enough in your mind, they can begin to drive the plot, sometimes in unexpected directions or at least with an unexpected tone or focus.