There comes a point when you're writing a novel, if you're lucky, when the characters start to wander off in different directions. Different, that is, from the paths you've carefully laid out for them using your Plotter-matic 5000 (or A4 pages taped together). This has happened to me in three books now and always at a stage where the characters are sufficiently well established to know their own minds.
I well recall attending a writing class; back in my youth when I had hair, optimism and aspirations (they all left on the same bus). One opinionated soul there declared with certainty that characters are only projections of the author and any notion of a book writing itself was just absurd. Maybe that's true for him but me and my characters laugh at him now, together.
So, is it a good sign? The characters becoming independent, not the laughter. Well, it adds another dimension and creates choices, leading to unexpected consequences that can impact the plot hugely.
In Covenant, my magical fantasy, two characters almost have sex and this leads to a revelation that became one of the core subplots. Whatever it did for the reader, this scene told me something I didn't know but needed to.
In Standpoint, a twisty turny thriller, my lead character Thomas Bladen told me about a past I didn't know he had - certainly not one I'd ever written for him. Later on, he spoke to other characters in situations I hadn't imagined and wrote the ending for himself. He also told me from the off that he was from North Yorkshire (I'd never been) and about his father. It was like meeting a new friend.
In Line of Sight, the sequel thriller still in the first draft, I'm 87,000 words in and one character has told me she's related to someone else. I was flummoxed and thought about ignoring her at first but it makes sense. Even if it does mean some rewriting to accommodate and resolve the plot line.
The point to this, I think, is that when we set up plausible, well-rounded characters, their choices and behaviour become independent of our well-orchestrated scheme of a book. It's often a revelation for the author which means there's a good chance it will delight or confound the reader too; and that can only be a good thing.
However, arguing with a lead character while driving, when he tells you where the book will end and how many weapons he needs, is I suspect something of a rarity. Still when he told me that he wanted a helicopter - and where to get it - to finish the book, he clearly understood it better than I had.