I think every script is different, the inspiration comes from different places, the ideas pop into your head in different ways, you come at it from different angles and start it in different places, and not always at the beginning!
I've written 8 feature scripts, 3 I would count as practice, 3 need some work and 2 I'm really happy with. Each one was a different experience. The first three I bulled into with out really knowing any of the rules of screen writing, without even knowing what I wanted to write or where the story was going. The next three I started to be more deliberate, take my time and think about the process and the last 2, I was meticulous, the last script I wrote has taken 14 months to complete.
In 1997 I thought I'd write a vampire road movie - friendly, misunderstood immortals who have learned to live without killing are found out and forced to go on the run with an old crusty vampire hunter, Midnight Run with Vampires basically. I still really like the idea, but I didn't execute the script very well, lack of experience led to lack of characterization. I was concentrating on the forward momentum, the atmosphere and the cool scenes and forgot aboutwho these characters were, essential for a story like this, would Midnight Run work without such highly developed characters? So it ended up flat.
I think knowing your characters is vital before you begin writing. Having a strong understanding of who your characters are will inform the story. You may have heard it said that characters will begin talking to you. It's true, they do, they'll say "I wouldn't do that" and when they do you know you've written a strong character and you know you can't bend them to the will of the story, you have to work out a different way to get them around.
I wrote a werewolf script a few years ago. I had a bunch of people trapped in a building and I knew how it would end. So I wrote that first. That was the third act. Then I had to figure out who all the character were, so I wrote about their lives before the werewolves. The was the first act. Then I had to join these two, second act. So that was ass-ways, quite literally! But it worked for that script. I don't think you should be afraid of writing whatever way it comes to you. Just get it down.
My other scripts were more straight forward, traditional I suppose, where I spent many months thinking about them, writing notes, ideas, structures, until eventually I get to the stage where I can start to plot it out by beats. Then I sit down to write. Because of those months of gestation, sometimes years on certain ideas, I'm able to blast out the first draft in a week. The werewolf script was written in 3 days (didn't sleep much). Then the writing begins.
For me the first draft isn't really a draft, it's the idea, as a whole vomited out onto the page, an I mean vomited, it's messy and stinks a bit! I have to clean it up, make it work, find structure and pace and make sure each character is working and has a singular voice. If there are characters in their who are just filler, just there to die, or say a funny line, they're gone. If there are two or three characters who's lines are strangely interchangeable then I fold them into one character and give him/her all the lines, and character traits. I find those character become much more interesting, and real!
From there everything gets worked on, teased out, every scene is questioned, pushed and pulled in every direction to see if it works. If it does then the challenge is to push every scene to be the best it can be, to give it everything you can. James Stewart often said that a movie is about creating moments. Each films should have several special moments, scenes that stand out, set the tone of the piece, make people think, gasp, react. Your job then is to find those moments with in the script and make sure they work.
This can take a while, for me usually about 3 months. Although the script I have just finished took 14 months! Be patient, work it out, make it the best you can. There are so many filmmakers out there who are so eager to get behind the camera and start shooting that they forget the most important thing, to get the story right, get the script right. I know there are been people out there that don't use like that, Mike Liegh, Ken Loach, Shane Meadows, even someone like Larry David who writes treatments rather then scripts and the cast ad-lib around the story. But these are extremely talented, experienced professional who've been working at the craft for decades, and they all began with an understanding of the craft of writing and how to tell a story.
You've heard the old sayings 'God is in the details', or, 'the Devil is the in the details'... everything is in the details. Why do the Coen brother's films work so well? They concentrate on the details. Everything gets looked at, from design, pacing, photography and so on, which all change film to film to illuminate the story, to language, accents, clothes and mannerism of their characters. They build a world. I read recently, in Sight and Sound, when William Friedkin was remaking the classic "The Wages of Fear" as "The Sorcerer" he asked Henri-George Clouzot what his secret was with the pacing of the film, Clouzot only responded "The Details", may not seem like much, but it's everything! - Friedkin didn't listen! The Sorcerer was a disaster!
I think it's an essential part of screenwriting, because you're basically asking you audience to live in your world for an hour and a half to two hours. So they had better believe. Personally I think that's the enjoyment of writing, is getting into the mind of your characters and the details of the world you're creating, or retelling. I think once you find that enjoyment you'll find your voice and getting to the end will be that much easier - well, perhaps easier is the wrong word, you'll have more tools for the job, more weapons for the battle.
I think all the books are fine, I've read quite a few of them. None of them really taught me how to write a screenplay, but they fed my passion, and I think that's just as important. What really taught me how to write screenplays was just writing writing writing and never stopping. I write all the time. I have 8 features as I mentioned, but 10 more I'm thinking about or fiddling with, a ton of shorts, TV show scripts and two novels. I do it because I love it, and that's half the battle. If you love writing, if you can't wait to sit down in front of the computer of get the notebook out, then you'll get there.
But you need to be writing all the time. It's a muscle, like anything, we're athletes of the mind! If we're not training everyday then we're going to get flabby and stiff and slow and lazy and before we know it we're sat in front of Cash in the Attic and Doctor Phil and finding a way to call it research! It's not.
I've told this story before: a friend of mine, had a meeting with David Keopp once, Keopp asked how long he had been writing and how many screenplays he had written, he proudly answered 3 years and 3 feature scripts. Keopp said he should be on his 8th script by now. So this friend went off and started writing like crazy, wrote 3 scripts in the next 9 months. Eventually he broke through with a show called Psychos, then came Spooks and soon after Shooting Dogs, plus two published novels. He's told me time and time again, there's no secret, it's just plain hard work, writing writing writing. Finish the script you're on, just finish it and move on to the next one.
I'd say don't get too hung up on it if it's your first feature script, get it written for sure, make it the best you can and get it out there, into the hands of producers. But keep writing, start the next script too. And if you find you're stuck, or you have writers block, I find the best thing to do is to move onto something else. Don't waste the energy waiting around getting frustrated. Start a new project, keep writing. It'll help blow out the cobwebs and you'll find the anxiety from the other script will be lifted and eventually the wheels will start rolling again. Prime example often given, the Coen brother wrote Barton Fink in the middle of Miller's Crossing because they had written themselves into a corner and didn't know how to get out. So they side stepped into another corner. Distance and time often solve many problems.
Happy writing!