Ten Things to Give Your Characters More Character
Birthday: Unless circumstances prevent it, decide on a date of birth for your character. Research his or her respective astrological sign, any famous people born on or historic events that occurred on the same day, and incorporate into your characterization some aspects that fit well and appeal to you. Create one event that is significant to the character that happened on one of their birthdays.
Collection: Almost everyone in real life collects something, so why not choose a collectible item tailored to your character's personality? Or give your character a hobby that creates a collection of handmade items. Decide how your character feels about their collection, how the collection reflects an aspect of their personality, and work it into the story accordingly.
Fan Items: This is a sub-class of the collectible, only something that is definitely from the realm of RL fandom. For modern novels, think about big fandom trends (Star Wars, Star Trek, Harry Potter, etc.) If you're writing a story set in a historic period, check out what forms of fandom there were in that time. A character who wears a Frak Me T-shirt makes a certain statement about themselves as well as their love for Battlestar Galactica. So does a character who collects death masks of famous Victorian poets.
Habit: Characters who have habits often reveal through them interesting personality quirks. These can be simple rituals, superstitions or largely unconscious repetitive behaviors, such as always checking all the doors and windows before going to bed, avoiding walking under a ladder or tapping a foot when they're feeling impatient. Think about your character's daily routine, and see where a habit might logically form.
Journal: I don't know many girls who didn't keep a diary when they were teens, and certainly plenty of adults of both genders keep personal journals. Characters who aren't the writer type often chronicle their lives in other ways, such as with home videos, photo albums or saved letters.
Library: I love books where I get to see what a character likes to read. You don't have to set up an in-house library, either; maybe toss a couple paperbacks on the nightstand. Who and what your character reads always says a lot about them to your reader, especially if they've read the same books.
Lucky Charm: Not every character is superstitious enough to hook a rabbit's foot on their key ring, but most of us have little good luck charms (I carry a pair of engraved stones in my pocket, and have another pair in my car.) A lucky charm can be a habit or ritual as well as a physical object.
Music: What sort of music does your character listen to? Do they hum along with the radio while driving, or sing in the shower? If you choose a type of music for your character that you don't listen to personally, check out some albums, read up on what fans of the music have to say about it, and create a musical persona based on your research.
Pet: Unless your character has allergies or is living under circumstances that don't allow pet ownership, incorporating a pet into the story helps the reader know more about your character through their need or desire for animal companionship. It's also fun to decide if your character is a dog, cat or other type pet owner and then give them a companion with their own personality quirks.
Vehicle: What sort of car does your character drive, and why? Are they sporty, sedanish or truck-minded? Did they settle for what they could afford, or save up to get the car they really wanted? What is your character's attitude toward their vehicle? What's in the back seat, the trunk, the glove compartment? For extra realism (if possible) see if you can arrange to borrow or test-drive the same vehicle your character owns, or get some insight from someone who has a RL version of it.