ScriptFrenzy prep going? Do you have your story? There's still plenty of time to get ready so you can join the big, crazy adventure of writing 100+ pages of script in 30 days.
Part of the background work for script-writing is creating your characters for the story. The more you know about your characters before you start writing, the easier it will be to write them. This is especially important in scripts since it's all dialogue; you need to know your characters to be able to make their conversations believable, after all.
I use the life interview in Atkinson's Gift of Stories as the foundation of my character biographies. It's quite extensive, and may not be needed for something like Screnzy, but it's a great resource for writers. You don't have to use that, of course. You should use whatever works best for you and your characters. Some people just jot a list of character traits for each character, some people write quick biographies, and I've known some people to invent journal entries for their characters to better get in their heads.
When you create your characters, you have to think about how they fit into your story. What is each character's role in the story? That will help guide your bios. For example, if you have a minor character who is only in a scene or two, you probably don't need to have a full background with family history, quirks, preferences, and what the character's house looks like. Conversely, if you have a character who is in nearly every scene, he or she is likely going to need to be more three-dimensional.
You don't have to write character biographies. Just like you don't have to outline. But it can help. It might make Screnzy go a little more smoothly. Take some time today and think about the characters in your story.