Actually, that happened to Margaret Mitchell. In her original drafts, Scarlett was a supporting character.
I think that happened to Tolkien as well. I think Smeagol forced himself into the story. In his letters books, he writes about this scene: This is when Smeagol comes back from making his deal with Shelob. From the novel: "For a fleeting moment, could one of the sleepers have seen him, they would have thought that they behold an old weary hobbit, shrunken by the years that had carried him far beyond his time, beyond friends and kin, and the fields and streams of his youth, an old starved pitiable thing."
Tolkien writes a letter explaining he had to make a tough decision at this point of the story. If Smeagol did repent, then the entire book would change and become about the redemption of Smeagol. That was not the book he set out to write, so he had to write the confrontation with Sam about "sneaking" to keep the book on track.
I think Tolkien was generally fond of Smeagol and it certainly seems to me that he was having a great time writing him - for he is one of the funniest characters in literature - so richly drawn that he is one I looked forward to seeing the most in the movies (and Jackson did not disappoint).
In the novel I am working on now, I have to watch the Jeb Stuart character. He is just so much fun to write that if I'm not careful, the book will become about him.
I have a character like that in my screenplay. About two-thirds of the way through Act II I physically ship Oswald away and out of the setting, for just that reason. (Of course the putative reason is that Offa had died and therefore Oswald is no longer persona non grata at the Mercian court.)
That’s an astute comment about Smeagol/Gollum.
Truly you can have minor characters who run away with the story.
The nest balance I knowq between contrasting chracters is the Aubrey Maturin series. Both Jack and Stephen hold your interest. Jack being the hero and Stephen the anti-hero.