We want our characters to be relateable but we know "normal" or "average" can be the death knell of boring in fiction. For me, it works to focus on their universality. What are the notes of that character that will resonate with every man, every woman, every person in every walk of life? Characters who have lived universal truths of love and loss, beginnings and endings, injustice and vulnerability, are at the heart of the reader's attachment to a story. Beyond that, I give them space to take flight into the quirky, abnormal, extreme or larger-than-life arena that makes them memorable.
Not so long ago it was unheard of in the romance genre to have a nerdy or blind hero, or an overweight or physically-challenged romance heroine, but authors like Vicki Lewis Thompson and Jude Devereaux and LaVyrle Spencer found success in the universality of those characters.
I have a book I use at this stage of writing: Building Believable Characters by Marc McCutcheon. Mark has been my go-to writing guide BFF since I picked up his Writer's Guide to Everyday Life in the 1800s. He adds some amazing tools to the writer's toolbox with his reference books. His Building Believable Characters book contains a character thesaurus of not only superficial lets-get-to-know-our-characters physical attributes but goes deeper into personality traits, bad habits/vices, diseases and psychological afflictions, hobbies, patterns of speech and even gestures and facial expressions when the creative well is running dry. It's a great source of ideas to help your character "take flight."
What's the most bizarre character trait or quirk you've ever read in a novel?