I think everyone's had that moment where an idea came to them and they said, 'hey, that would be such a great idea for a movie/book.'
Equally, it's probably true that most good works of art, whether they are movies/books/tv-shows/paintings/songs... started out that way--with a basic premise that inspired their creators into action. So when we casually have such an idea, it's quite appealing to believe that those thoughts could be a real and glorious thing just like all of the finished projects out there that we admire. Unfortunately, most of the time, these little kernels never turn into popcorn. With regard to books, since that's the theme of this blog, some people make the effort to sit down and turn their idea into a project and fail. That's because they have no process--no mechanism for developing their idea so that it becomes hundreds of ideas put together under one thematic roof. But this activity, like so many things, is actually a skill that can be practiced, taught, developed... accomplished writers pump out new projects quickly and on demand because they know how to efficiently get from point A ('that would be a great idea for a book') through to point B (the book has a setting/plot/characters/scenes/dialogue/everything else). So let's talk about what their process might look like.
Suppose your idea is: 'A secret agent has to save the world from a mad villain, and the government has given him a license to kill bad guys along the way.' If that's all there is to the premise, the author is going to need to flesh things out a fair bit before it becomes presentable as a James Bond story. It might not be too hard to characterize the main protagonist, main antagonist, people supporting each of those parties, etc. But you'll need to dream up a specific agenda for the villain so that the conflict can kick off from somewhere, then put your protagonist in at a particular chronological point. Complications will need to be added so that it isn't complete plain sailing for him. The events will need to be fed to the audience in such a way that they can follow progress, but other information will need to be withheld from them so that you maintain enough intrigue. There will be a host of little problems to solve along the way, like making the characters and their actions/dialogue believable, keeping everything consistent, making sure the audience can invest themselves emotionally and cerebrally, and so on.
If you're writing according to a formula, or copying your plot ideas from somewhere, it's a bit easier. But lets say you're starting from scratch, because you want the structure of your story to serve the ideas themselves, not the other way around. Here's how I might proceed.
- You decide on your central idea/premise. Let's say it's: 'During World War 2, a British man becomes a conscientious objector, and avoids joining the army.'
- The very next step is to identify the central conflict. That shouldn't be too hard: you need a character who represents the main opposing sentiment--someone that wants our protagonist to go to war against his wish. Maybe it's a close friend who has already signed on, or its someone responsible for recruitment. What if it's the guys own wife? That way the conflict isn't just ideological, it would inevitably be quite emotional and convoluted as well. I'll pick that option, bearing in mind that it's not the only choice.
- The situation our characters are in is going to have to change in order for the plot to work. Perhaps the story starts with him already having joined the army, but he becomes a deserter. Or, he initially avoids conscription, but later chooses to when he comes under pressure from his wife/everyone else. Suppose we go with the second option there.
- Now we can start looking at where the most important scenes are going to be. There will be one centerpiece of action where he gives in and decides to go to war. We can systematically zoom in on the situation and look at the crucial cause and effect of dialogue and people's resulting feelings.
- Having mapped out where the turning points are, we can build other scenes around them that effectively support the story by developing the situations and developing the characters so that the audience has the right expectations going into the pivotal scenes.
- This process is like building a house. You start with the fundamental parts of it, then add layers of detail around the central structure of it. You don't put everything together simultaneously, you start at a specific point and attach new things to it one by one.
Some people really prefer to write spontaneously. That's fine, because it will mean that they are writing at the point in time when they feel most inspired by their ideas. That approach can have its drawbacks though. We sometimes see writers add a lot of filler material while they are working towards an important scene, because they haven't properly decided how to support it. Or, the story is really light on ideas because they only picked one central idea and tried to blow it up like a balloon, instead of using it progenitively to populate the world of the story.
Having a more structured and systematic process can really be useful if you are willing to brainstorm a bit. That way, your good 'starting idea' can slowly but surely be turned into hundreds of other solid ideas that eventually get combined together according to an overall plan and result in an actual finished project.