I love reading books by authors who know how to create vivid, real, three-dimensional characters for me. I'm a big fan of plot twists, suspense, and humor, don't get me wrong, but if all of that is coupled with cardboard-cutout characters, I'm just not interested.
Loving three-dimensional characters and learning how to write them are two different things. Here are a few tips on how to do it well (please add your own in the comments section if you think of anything I've missed!):
1. Give your characters a past. Your character's life doesn't start on page one of your manuscript. He/she has friends, family, schooling, hobbies, first dates, injuries, lessons learned, camping trips, vacations, fears and reasons for those fears. Characters don't spring fully-formed into adulthood. They are shaped by their past and the choices they've made. You don't have to give every detail of that past in your writing but you should reference it comfortably to give your readers a sense of what formed your character.
2. Give your character a unique voice. I can't stress enough how important this is. Every character in your book should have responses, expressions, and thought patterns that are unique to them. Take the time to get into each character's head before writing their POV to make sure the words are authentic to that character. If you're worried you have characters who sound alike, highlight every character's dialogue or POV in a separate color and then compare.
3. Give your character flaws. Heroes and heroines who never exhibit poor reactions, wrong choices, out of control emotions, or Achilles heels are just annoying. Readers don't want pristine, never-makes-a-wrong move characters. They want characters they can love, flaws and all. Often a character's flaws are useful in driving the plot toward the Black Moment. Don't be afraid to let your characters make bad choices, over react, or nearly lose everything because they've got a blind spot the size of the Pacific.
4. Give your character room to grow. Let your characters start the book with something to learn, something to attain, some unbearable choices to make and see what happens. Throw in obstacles, yank out support, strip away their options until you, the character, and the reader can't see any way for a happily ever after to happen. Do all that and then see what your character is made of and let them cope.
Anything else you'd like to add?