Completing a story is a long process. As much as you are called to write the story, you will appreciate the time you spend developing your background. For your main characters—the hero, the heroine, the villain—the more you know them the better your story will read and play out.
It’s not enough to know your protagonist has blue eyes, is handsome (or not), and likes strawberries. Know his background; what Syd Field calls the Circle of Being. What happened when he was growing up that affected him permanently? Use these details to further your story.
I was recently reading a script for a client. The story had four main characters: two men, two women. Each one stated his/her knowledge, his or her preferences, etc. One was Asian, one was a blonde bombshell. One was big and strong. However, none of the stated facts actually advanced the story. The characters were as flat as Male 1 and Female 2 and all four were almost interchangeable.
Many writers interview their characters. Others create a long description of what they look like, how they respond to tension and crisis, how they eat, etc. Use whatever detail development process works for you. You will probably end up with ten to twenty pages for your main characters. Then use the particulars in scenes to enliven the character and advance the conflict.
The process is involved and time-consuming. But, in the end your story will come alive.