Your personal Tumblr library awaits
Get to really know your character. From novel to OC in a fanfic, know every little thing about them. What is their favorite color? Most hated food? Deepest fear? Guilty pleasure? Are they afraid of heights or spiders? If you don't know them intimately, you can't write them well. It's like trying to tell the life story of a stranger you pass on the streets. You can't explain why they punched the wall, but they did. You can't write a character that people will get attached to them if you aren't attached yourself. A fun way to figure out their nuances is take those silly personality quizzes online. They actually make you think really hard on the little stuff and it helps big time. Remember that the world around them will also affect things. If a war is going on, the most likely won't be eating like kings. Do they resent it or is it all they've know? Sometimes it's the little things that really open up a character. It can be as stressful as trying to pull all the skeletons from your best friend's closet, but it's worth it in the end. (I.E. had a character afraid of the color orange and they were stuck in a pumpkin patch with their friends. Panic ensued and fighting happened, all because it was a feared color.) Any detail can seem too little to include, yet they can be the driving force behind a scene. Oh, and keep notes on their personality and preferences. It's easy to forget it all when you set down your writing.