Have an idea of where your story is going. Does it take place in a day or does it expand over decades? This information will affect the type of people your characters will be.
Decide on the type of characters you want in your script. Be thorough about this decision. You need to know everything from age to race to religious affiliation to whether their parents are divorced and everything in between.
Research your characters. Once you have an idea of who they are and where they are going, give them realistic representations in your script. If you're a white, middle-class, Protestant, male, don't assume you know enough about the life of an immigrant, twenty-something, poor Latina to write a movie about her without conducting the necessary research.
Writing a character bible is about knowing everything about your characters and that includes their family. You don't necessarily have to create a separate bio for the family members but you must be clear on where your character fits into their family and the relationship they have with them.
Include a basic summary of your characters for each one you write. The basic summary should include age, weight height, sex, schooling, occupation, where they are originally from and where they currently live.
Write a detailed biography of each character using the information you've accumulated in Steps 1 - 5.