Most of my novels start out with either a plot idea or a setting. I rarely begin a story line with a specific character in mind. This is opposite of what a lot of authors do but for me, the story unfolds and the people in the story take shape with the plot line. May bee because of this, often my characters are difficult to flesh out.
A character must be so real to the writer that it should feel like you're creating a child. You must know their every thought, desire, fear and pet-peeve. You have to know if they are a dog or cat person. You have to know where and if they went to school. Who they liked, loved and hated throughout their life. You must know what they would say in any given situation, why they would say it and whether or not their being honest or covering up their real feelings.
Often an author knows their characters better than they know themselves. This is an intimate, deep and all-encompassing endeavor and can be exhausting and thrilling.It also creates a situation where hundreds of pages are often deleted while the writer comes to know their character. I hate to delete my work. I hate to delve so deeply into a character and then never tell the story that made me so well-informed about who they are. The truth, though, is it has to be done.
So instead of telling you to grit your teeth and bare it, I have another tool that works for me...
Write short stories about your secondary characters as they interact with your POV, or Point of View, character. Write these stories from the secondary character's mind and eyes. It will show you what your POV character looks like, sounds like and how they come across to other people. It will also provide side story lines and series potential for your novels.
I have three books I'm either working on, developing, or re-writing and these secondary character story lines give me the chance to develop other characters, take a break from the intensity of my POV characters and go down side lines with characters I love.
In the meantime, keep writing, reading and developing your characters.