Okey-dokey ... so, your premise is being honed, refined and polished. You've got your basic set-up, you're paying attention to the genre and you've given some thought to the most appropriate format for the piece.
You might even have a title (but don't get too attached to it just yet).
The development of your project now takes place along parallel lines.
On the one hand, you'd be well advised to build your basic premise up in stages, aiming first for a half-page synopsis, then maybe a synopsis of a page or two, maybe making it to a five or six page outline after a little while.
This is how a great deal of script development is carried out in television. It's a simple Cartesian process, starting with the most basic 'synopsis' - the premise - and progressing to ever more detailed outlines.
('Synopsis' and 'outline' can mean the same thing; I tend to call the earlier stages synopses, and the more detailed story work outlines - the synopsis summarises the story, the outline goes into more plot detail.)
We'll worry about treatments separately. They are a blessing and a curse, and will need to be looked at shortly.
For now, though, what do you need to be thinking about as you build up your story from the premise, through a synopsis or two, to an outline?
Let's start with your characters. You've already established who your main character is - the protagonist who wants something but will have to overcome obstacles to achieve or acquire it.
What about the other principal characters?
Have you begun to establish a clear antagonist? A villain, rival, nemesis or shadow?
Has your hero got a side-kick, a friend, ally or partner?
Is there a love interest anywhere in sight?
Not all stories need all four characters (HERO, SHADOW, SIDE-KICK and LOVE INTEREST). What is more, a character can quite easily shift from one role to another. A side-kick can turn into a love interest, or even into a rival or shadow. A love interest can become an enemy.
Giving some thought to these central characters at this stage is helpful. Too often, scripts have quite strong central characters, partly because the writer identifies with the hero, but the satellite characters are two-dimensional cardboard cut-outs who exist only to advance the plot or do what the hero requires them to do.
Urgh! Avoid that at all costs. The simplest way to do so is to remember a fundamental fact:
WE ARE ALL THE HEROES OF OUR OWN STORIES.
Every character in a script, if they are going to feel at all real, must have his or her own agenda. Just like the hero, they want something, and something stands in their way.
So - while you're gradually building your story up from premise through synopsis to outline, do a little work on the side with your characters.
You don't need to fill out a whole questionnaire about each character, but you should know what each character wants and what's preventing them from getting it.
This is essential for plotting your detailed storyline. It's also fundamental to the drama of your script. If everything's too easy for your hero (because that's the only character you care about), then there's likely to be too little drama. The struggle's not there.
But if every major character is acting as if they were the hero in their own story, wanting things, struggling to achieve them, facing obstacles, making decisions and taking action, then you'll get conflict of interest, stronger characters and a better plot.
And never forget that character is revealed through action. Character is what character does. So make sure that your characters are able and willing to do things. They're not pliant or dormant. They're not just there to do a bit of talking. They're real people, with needs, desires, fears and foibles. And they DO things.