Okay, fellow writers, look back on your various works (especially the longer ones). Think about the characters you create. Did you ever find yourself writing a bit of dialogue you weren't expecting to write, or having them act in a way that you weren't planning on?
If no, then either you're the best predictor of human behavior, or you don't know how to make believable characters.
It's blunt, but it's true. The worst pieces I have ever written were always ones where my characters did exactly what I expected them to. The less believable a character is, the easier it is to mold them to do whatever you want them to do.
My biggest problem with this particular manuscript is that the characters all walk the fine line between hero and anti-hero, villain and misunderstood antagonist. No one is 100% right or 100% wrong. Y'know, like real people. This has resulted in scenes getting written and rewritten, dialogue changing before I can even type the end quotation mark. The only characters I have no issues writing are the ones that are meant to be caricatures. And, in the writing world, a caricature is a walking talking mannequin, only there to prove a point or provide a laugh.
My main character has changed in personality about 40 times since I first came up with the idea. And she's starting to do things that I wasn't planning on.
Writing characters is a lot like raising children. You can only do so much before they go off on do their own thing. Which is okay. They're being believable human beings having believable reactions to things. And if they're believable enough to you that you find yourself writing things you weren't expecting to write, then there's a chance your readers will find them just as believable.