The place I always start with my character is their physical appearance. Why? It's the easiest to imagine. So you want to start with general descriptions (height, weight, body type, hair color, eye color). But once the body starts to take shape you want to really start fleshing your character out with details that make them more real (do they have acne, bad breath, scars, tattoos, piercings, right handed or left? Do they slouch?) Do you see how an interesting story can start to take shape now? Maybe your character has a scar that could change the course of the entire novel (hello Harry Potter).
Next question to ask is what does your character want more than anything in the world? This will directly lead to a conflict. Now when I say what they want more than anything in the world, I don't mean a small time goal. You could write a novel about a character getting a promotion and something is getting in their way but is their ultimate ambition to get that promotion or is it something else?
Next ask what is your character's greatest fear? Don't be afraid to have fun with this. It could be something as common as a fear of heights, to something a little more obscure like they are afraid to live anywhere without concrete because it might be quicksand.
What are your character's hobbies? It's important to create a whole new world when you're writing a novel. It doesn't matter if your character is passionate about music, do they play any instruments? Are they in an ultimate Frisbee league? Maybe your character loves training for the July 4th hot dog eating competition their neighborhood holds every year.
And then finally get to some of the small details that might not make an appearance but are always nice to know. Favorite color? What kind of music do they listen to? What's their favorite movie and TV show? Do they have a lot of money? How did they get it, trust fund, invention, robbery?
You can never know your character too well. The more you ask about them, the more they will answer you. Think of it as a first date. Don't be shy? And once you list out all your answers try writing a 4-5 paragraph biography on your protagonist to tie it all together and find their voice.
Finally, do not just have a dynamic protagonist and then a bunch of cut out, predictable characters for supporting cast. Get to know as many characters as possible. Not necessarily every single one, but enough to keep the story feeling real.
I hope this helped.
Thank you for reading!