1. Re-draft your story several times. Most often what happens during this 'fine-tuning' process is that you build on your characters. Mine have the bare bones in draft one and usually by draft 4 I think, "There you are!" Although, there are always exceptions when you have a strong character
2. Create a bio. Start out with the basics on looks. Then have categories on personality, background (even if no one ever knows this part, it helps you figure out why they are the way they are), style of clothing, habits, likes, dislikes, favorite foods, favorite phrases, etc.
3. Have an Interview with your character. Write up a list of questions that you might to get to know someone and whether you write it down or just talk it aloud, go through this with each of your characters.
4. If your character is foreign, try to use colorful phrases from where they're from. I wouldn't go nuts with slang, but how you write them may also depend on other factors. Since mine have also lived in other countries, and America, some of that has rubbed off on them. So they may not adhere to using strictly UK terms for things, and that allows me a little freedom to not screw it up as an American writer. But sometimes giving them their own little word or phrase can be endearing. Unfortunately, Christian Poe's is fecking. Not sure how endearing that is. Not that he says it a lot, but it's his thing.
5. Have your characters doing things. Give them habits. Adam likes to put his hand up on the wall and lean on it, Simon tends to pace and talk a little frantically, Christian enjoys candy but has a thing about littering, Justus rubs his tat a lot, Logan stands with his arms at his side and doesn't often fidget or fold his arms. He also has a thing about putting his fist to his mouth and hiding a laugh, but something we only see him do with Silver.
The fun part about characters is breathing them to life. Instead of having Jack fold his arms and look at her punishingly, have him put his hands in his pockets and jingle the coins in there. Or give someone a Talisman they carry around, or perhaps they have a thing about always eating almonds. Watch people in real life and pay attention, or go on the Internet and look up body language. I had an interesting site bookmarked that showed the body language of men and what that meant, so I've used it.
This is what put me on pause for the sequel of Book White. I've got my characters, but one of them I'm struggling with figuring out who he is. I need to have a long talk with him and figure it out, because I can't continue with the book until I know. I lack time at the moment, so it's a future project. The first book which I'm having edited this summer was effortless. The girl was so vivid in my head, as was the guy, but it did take a few attempts before I felt like I nailed them, and really "got" where they were coming from.
So few people want to read about that dramatic Vampire who stared at you with dead eyes and moves with grace. Give him the hiccups when he's mad. Let him tell a story about something that moved him. Have his favorite word be "sniveling".
Sometimes as we delve into a series, we learn more about our characters in books 3 or 4. I have no problems adding on new personality traits, because often that's how it works in real life. When I first meet someone, they give first impressions. But a year later, I start noticing other things I didn't before. This is what happened with Novis, mostly because we haven't seen much of him in previous books. We get a few more interactions with him in Book 4 and I created a very amusing little habit he has - it's just his thing.
So, that's the process for me. Hope as a writer or reader it gives a little insight. When you put a book down and can't go through with it, think about why. Sometimes it's the story or writing style, but often times it's because nothing about that character really stepped out.
This is an example I copied straight from Justus's bio that I keep;
CLOTHING: Wears a gold ring on right hand symbolizing he works for HALO and it has etchings. Always dresses in fine, expensive, tailored clothes or suits. Even casual clothes are expensive. Likes to wear watches even though Mage can tell time naturally. standard workout attire: black pants, tank top, and barefoot.
FOOD/DRINK of choice – Drinks tequila straight sometimes. Enjoys fine wines, expensive stuff. Doesn’t cook, but loves to eat manly food like ribs, meat, and messy eater, eats with hands or spills food, not so much refined with silverware