New authors have a bad habit of writing characters into their stories as though each character is a single emotion come to life. The good guy is just good. He's unshakable, he's unbreakable, he has nothing to lose, and so cannot be beaten. The villain is evil beyond darkness, and fears nothing. He too is unshakable, and unstoppable, and he hates the good guy. The girl is madly in love with one, or the other, or both, but has no personality beyond that love (cough, cough, Bella Swan), and the cast of friends are only present to support, or bring down the protagonist/antagonist/etcetera.
Longing gazes, and long love stricken soliloquies are fine for poorly written teenage romance novels, wracked in strange fantasies, and marble skinned vampiric sexuality, and it certainly will make a published author rich... but I would rather be a respected, poor writer, than a rich hack... and yes. I have a personal-professional problem with a certain Twilight author. It is because of her that your work has expectation built upon it now. God help you if you loved Bram Stoker's Dracula, or Anne Rice's vampires, because if they don't glitter like diamonds in the sun, well. Let's just hope your characters are compelling.
Which leads happily to the next part of this lesson.
The compelling character is a character that has hopes, dreams, sins, and saving graces. He, or she, has memorable qualities, and weird quirks. Your tough talking character does not have to have adamantium claws, and a cigar. It's very one dimensional, because let's face it: He's the best at what he does, and what he does isn't very nice (I actually love Wolverine, but still: He was created for one genre. Comics. His character doesn't translate well into three dimensions, because he is the thesis of personal loss, pain, suffering, and revenge). Your anti hero can be a rough and tumble bad ass, who is also a rough and tumble homosexual (IE Agent Smecker in Boon Dock Saints). Your sassy sweet heroine doesn't have to be Thelma, or Louise. She doesn't have to be Joan Beaver, Mrs. Brady, or Mrs. Robinson.
TOO often are we focusing on media icons, modeling our characters after people like Jolie, Pitt, Cruise, Combs, Smalls, Pac, etcetera. We take what we see in contemporary society, and apply it to our work, and we create these characters that read more like D-lebrities, rather than people.
Three dimensions means your character eats, sleeps, and uses the restroom. They brush their teeth, comb their hair, they enjoy whatever things people enjoy, because they too are people. If your character has a job, and doesn't go to work, they had better have a doctor's note, or they're getting fired. If they have kids, those kids had better go to school, or a truant officer, or worse, child protective services will be breaking down their door. This is just the reality of it. You should translate your knowledge of a character into written form without having to slide into exposition. Likewise, you should be like the FBI with your characters, having complete files on their existence, transactions, lifestyle, boyfriends, girlfriends, dogs, cats, cars, and extended family. If your character were real, they should be terrified, or extremely angry that you know so much.
This knowledge helps you as you write, fills in the blanks, and allows you flexibility, and reference as your story progresses.
I can't help you decide who your characters are, or how they behave. I cannot tell you their names, or ages, or why they are the way they are. I can only tell you that they are NOT abstract pieces of multiple personalities, based on single emotions, and acting in that category only (unless you're actually writing a story where the characters are EXACTLY that).
I can only tell you that once you create your character, he, or she, should be someone that another character, or even you, could fall in love with. Someone that others, or you, might love, or hate, or argue with. Someone who should put down the toilet seat but always forgets, or close the lid on the pickle jar. They are a person with virtues, and sins, pride, and humility. They are good, and evil, and everything in between... and that is what makes them real. They are born of you, but grow on their own, so nurture them. They mean the success of your work, and without them, your stories would be nothing more than a road-map: a scenic guide to what could have been.