Repeat this to yourself: Characters are easy! Only real people are hard.
(Can you see where my current problems lie?)
While real life people are hard to decipher, step in where they aren’t wanted, and insistent on not following the plot, characters can easily be whipped into submission. Well, more easily than real life people.
And that’s your job, writer. It is your wonderful, creative, and fulfilling job to discover who your character is, why she acts as she does, where she needs to go, and how she can work with the plot (even when that means that she’s working against the stated goal).
Your goal is to know everything about your character from the minute of her birth to the sound of her laugh to the real reason why she hates her nose. It is impossible to know everything, but it’s a lofty goal that you must have.
Because your reader demands it.
If you don’t know your character, your reader will be left with questions that can’t be answered. Not questions that can be answered multiple ways, as some stories are meant to be read, but questions that cannot be answered at all.
The result of this lack of character is that the reader will not “suspend disbelief” to follow your character on her journey, and will not read your book!
Let me try to be less abstruse with an example.
Your character is ugly, stupid, crass, not as rich as those around her, not from the same background, not skilled, half-crazy, overly-confident, creepy, acts either like a 14 year old or a 70 year old and is neither of those ages, has a job she never goes to, is combative, is uncompromising, hates everyone, and is constantly getting in the way of everyone. Your character has no redeeming characteristics. (This character is not based on a real life person. I promise.)
Now, let me ask you: Why would your reader believe that a classy, rich, kind, mature, sensitive, and gifted man would marry her? Why would your reader believe that this person has friends, let alone friends who are willing to go along with her to their own detriment? Why would a group of people put up with this bore when they are all wealthy blue-bloods who aren’t used to putting up with anyone beneath them?
If your reader cannot believe what you’ve written, they will not finish reading your book, they will never buy another one of your books, and they will bad mouth your book and you in reviews. And that’s if you can get someone to publish you.
But maybe your character is the same save for one characteristic. Maybe the character is disabled and pitied by others, maybe the character is a celebrity and people hope that she will live up to her film persona, maybe she’s grown a tumor and doesn’t act like she used to but people ignore it, or maybe you change one characteristic (make her gorgeous, or fabulously wealthy, or a drug dealer or something) that will make it believable that others would have her around.
Your character doesn’t have to be likeable, but she must be believable within the scope of her world.
Just remember: Characters are easy! You can change them! They can grow!
Unfortunately, real people are not so malleable.