Simple plot. Complex Characters.
That's a rule I live by.
That, and that nutella is one of the best breakfast foods.
Character development was a tricky thing for me to learn, and I'm still growing in that area. (My own character development, if you will.) More often than not, all my characters ended up sounding similar to each other, so I began researching relentlessly on how to develop complex characters that readers connect with, and here's what I found!
Character relationship are important. At a basic level, every side character is going to have a relationship with the main character. If we use Harry Potter, you'll notice Harry has a relationship with Ron and Hermione and Hagrid and Dumbledore, etc. His relationship with each person is different (as it should be.)
But for true story depth, you can't stop there. Those side characters all have independent relationships with each other. Hermione has a relationship with Ron. They each have a relationship with Hagrid. And so on.
You don't need to bring out every single relationship in your book but by giving secondary characters their own conflicts/relationships with someone who isn't the main character, you immediately add depth to your narrative by taking things to the next level. (-Advice from best selling author of A Curse So Dark and Lonely Brigid Kemmerer.)
When describing your characters, avoid generalizing. Tall and handsome means nothing. I know what handsome is, but I can't see it. It's your job to make me see it. Now, if you tell me "wrinkles lined her face, and one eye sagged a litter deeper than the other," or "as he smiled, his mouth almost swallowed his ears, and the little scar next to his eye bent toward his bushy brows," those tiny details are descriptive enough that my mind will fill in the rest. Find a specific detail about their looks, then remind us a few times through the book of that one detail, and we will have a better idea of what they look like than if you say tall, brown hair, handsome strong face.
Continuing off that, avoid making your characters too pretty. I can't relate to someone who is physically flawless. It actually irks me a bit when the characters are all too perfect (A court of thorns and roses.) If you make all your characters strong and handsome or thin without even trying with thick hair that smells like flowers, I'm going to hate them. Those people aren't real. Give me real people with real flaws, both physical and personal.
Some physical flaw ideas are a crooked or long nose, thin lips (me), a blemish on their face (also me), bent ears or ears that stick out (seriously, all me), gap in teeth, wayward hair, limp, bitten nails, wrinkled clothes, or a large mole on their arm. I get that these can be endearing, and for the right person, they will love that gap between your teeth, but it still makes you human.
This next one is huge. Characters should grow in the book. Give your top five, yes, FIVE characters a personal goal, and everything they do should be in pursuit of that goal. Once you know their goal, give them character flaws and opportunities to make mistakes, so they have someplace to grow from. The best characters are the ones who have changed by the end of the book, or else the entire adventure was for naught. Again, THIS IS CRUCIAL! Characters should change, either for the better or worse, through the plot.
Give your characters a unique way of speaking. When surprised, one character may say things like "Whiskers and Willerbknees!" Or one character only says a few words, or one often ends with saying "Yup yup!" These little quirks add depth to your characters.
To help create real characters, base them off people you know, even if they are people from a tv show. This gives you a concrete image of who they are and how they would react to situations, and writing them will become easier!
I hope this helped you as much as it helped me! Can't wait to see what lovely characters you all create!