As a writer the one thing I spend most of my time worrying about is... well, if I'm going to succeed or fail in the business. But aside from that, it's the initial idea stage, knowing that right now you're staring at a blank piece of paper, onto which you could write anything you want. Oh, the responsibility! Ideas, ideas; but I'll be damned if they're anything but goodideas.
Ideas come in all shapes and sizes;-
First of you have the colossal idea. That's an idea that when you first think it up (or more appropriately, realise it) simply blows you away. Now this is usually the main premise of the film - one that's often high concept, and a grand starting point for the rest of the ideas (characters, motivations, cause/effect). These ideas are two a penny, and it's the writer's job to fish out the very best, and once you've twisted and turned them enough, to use them as the main anchor for what follows. In the case of The Other there's two colossal ideas - 1) Let's make a sequel of sorts to an already completed film!, and 2) Using the emotional journey I went through after the death of a friend to project onto a story about the passage of child to adult.
Secondly, you have the bus ride idea. These ideas follow on from that initial colossal idea, and emerge from your subconscious mind. You, as writer, can just be sat down doing nothing (especially not thinking about your colassal idea) when thoughts will just pop into your head. You'll start to picture the world of your script, the characters that occupy it, and eventually what it is that these characters want (and then do to get those wants). Usually for me these ideas hit home when I'm - ironically - on the bus ride home, through listening in on conversations, and picking up minute details about people's lives and using these observations to serve one's own purpose. Oh, that little girl just got a nice pink bike; what if Horatio was to ride a pink bike? Or maybe steal one? You get the picture.
Next up, narrative ideas. When one has imagined his/her imaginary world enough to be fluent in its environment, then one is ready to write about the world in detail. But it's not enough to simply write the character's lives - there needs to be structure, or else everything fails. The audience can't relate to story, and the character's emotional cycles will not 'read' well, or the audience won't invest in them suffiecently enough for it to matter anyroad. Narrative ideas are, in short, the way the writer structures character progression throughout the script, and uses this structure to effectively tell a decent story. Beginning, middle and end may be the most traditional, but at times that format bores me rigid. Think up inventive ways to tell the story! Keep the audience guessing, and therefore on their toes. Use their intellect and allow them to piece together what's going on, and then they'll forge their own ideas.
Note, it is not as scary as it seems if the audience have ideas of their own, and bring it to your text. Honest!
Then there's evolutionary ideas, when the script leaves the writer and for the first time others get their grubby mits on what it is that you've written. Costume designers will think that the character suits black, because it matches the character's enigmatic persona. Lighting directors will cast the bad guy in shadow; or the good guy, if the traditional pro/antagonist roles have reversed. The director will want to do this, do that, etc, etc. These are all good ideas, and are needed if the script is to evolve and survive. Other people's input=good. Look at it as a filter, getting rid of what might not have worked in your draft, but - hopefully! - keeping the great points for future reference.
These ideas continues beyond the giving out of the shooting scripts. The actors will make choices based off of ideas that they have gathered together about your work, and once everything is show, the editor will again re-evaluate the story and its ideas, and evolve it through reassembly. The evolution of the central idea is constant, right through to the end of production, and up to the point where the audience finally sees what has been produced.
The audience's viewing sparks ideas, through interpretation and these ultimate decide if a text is enjoyed, or dismissed. Of all these ideas, this is the one the writer has least control over. Hell, he or she has no control over what their audience thinks. Not at all. And that's exactly how it should be; otherwise we'd live in a civilisation leads by writers turned dictators!
Finally, there's plain old stupid ideas - like setting a court room drama on a plane, or having a blind protagonist decide he wants to be a critic of silent movies in the 1910's, or casting Paris Hilton in your flick. Stupid ideas, that'll you'll no doubt still have, but NEED TO IGNORE! If you don't, and find yourself writing what those silly ideas have implanted upon your brain, your script will no doubt fail. Or end up directed by Uwe Boll. Your choice.
So remember; let the ideas feed your imagination, and at times, lead your writing to where it needs to be. Don't try to control them, or they'll get away from you forever. Ideas are free, forever echoing around the grand cosmos of our's, ready for the right person with the right mind to catch them, and use them for good.