Before I sit down to write a script, I do an outline. But before that, I like to write a character portraits. Not every character gets one – for the script I’ve just finished, it was the hero, his sister, his love-interest, the rival love-interest and the bad guy.
As for how in-depth they have to be, there’s differing views on this. Some people like to do mini books on each character, covering everything from how long they were breast-fed to who their first crush was and when they got their period. And most important, how they felt about each of these things.
My feeling is that you can know a bit too much sometimes. I once had a panic attack in a school history exam and had to be taken outside to breathe into a paper bag. It wasn’t that I didn’t know my stuff – I just knew way too much to write down on paper in two hours. And scripts are no different.
You should know enough about your characters to be able to give them life and depth but not enough to make them seem like people from a novel. You have 110 pages to tell us everything we need to know about them – most of which must be subtext rather than straight-up description. There just isn’t enough space to go into their childhood peeing problems, unless it directly impacts on their story in the script.
I think a good template for a character portrait is this one here, which gives a list of questions to ask about your main characters (where do they live, what’s their job, what’s their greatest need?). Ask these questions and you should know enough about your hero to write a compelling story but not so much that you end up bogged down in massive detail.