Hi, let's get started. You can plot a novel down to the last chapter and still lose readers on page ten, because plot isn't what keeps them turning pages.
It's whether they believe the people in the story. The good news is that convincing characters aren't a mystery.
Classic novels have been showing writers how to do it for a couple of centuries, and the techniques are right there to borrow.
Flat characters do one thing consistently. Convincing ones hold two opposing traits at once. In Pride and Prejudice, Mr. Darcy is arrogant at the ball and quietly generous with his tenants. That gap is what makes you want to know him. When you build a character, don't just pick a virtue and a flaw. Let the flaw undercut the virtue in a way that shows up in scenes.
You can describe a character for three pages and still say nothing. Jane Austen barely describes Elizabeth Bennet's face. Instead she shows her walking three miles across muddy fields to visit a sick sister, and you understand her stubbornness in one action. Don't tell readers your character is brave or selfish. Put them in a spot where they have to choose, and let the choice do the talking.
Convincing people come from somewhere. In Great Expectations, Pip's shame about his blacksmith background drives nearly everything he does. That detail, a specific trade in a specific place, makes his ambition believable. Give your character a job, a hometown, a habit of speech. The more concrete the roots, the more real the person feels when they get pulled up.
A character with no desire is a character readers can't follow. In Moby-Dick, Ahab's obsession with one whale carries the entire book, and you don't need to agree with him to keep reading. Your character's want doesn't have to be grand. It can be a small thing, like keeping a job or winning back a friend. But it has to be specific enough that readers can track whether they're getting closer or further away.
Some characters transform and some dig in, and both are convincing if the story earns it. Scrooge's shift in A Christmas Carol works because Dickens shows you the moments that break him open, one memory at a time. A character who refuses to change can be just as gripping, as long as you show the cost. Either way, don't skip the middle. Readers believe change when they've watched it happen.
How a character talks tells you who they are before the plot gets going. Huckleberry Finn's narration sounds like a boy talking to you, not a novelist performing. Read a classic novel out loud and notice how each character's sentences have their own rhythm. Then check your own draft. If every character sounds like you, you've got work to do.
None of this requires a grand backstory or a dramatic secret. It requires attention. Pick one character in your draft and give them a contradiction, a specific want, and one scene where a choice reveals who they really are. You'll be surprised how much more real they feel on the next read.