Have you ever had a situation where you look up from the novel you're writing and realise that the character you thought was the main one isn't getting as much of the limelight as you originally intended? It can happen so easily. A minor character with an interesting quirk starts to take over, or grabs attention, and it can be hard to stop.
It certainly happens to me. In Court of Dreams I had to work very hard to balance a relatively normal main character with a bunch of weird and wonderful minor ones. I think I got the balance right, but that will be up to the readers.
It even happens when I'm ghostwriting. In fact, it happens a lot when I'm ghostwriting, because the person providing the ideas will give me main characters ready made, and I'll find my interest caught by someone who isn't fully developed as a way to exercise creativity. Thankfully, I've learnt to use it as a way to produce fully rounded casts rather than just unbalanced books.
So, does it happen to you? Do you find minor characters suddenly become at least as interesting as main ones?