First of all I formulate a group of action ideas – the clashes that will happen and other problems and hurdles the Good Guys must encounter, not forgetting a few nasty ways the Bad Guys can work them over and/or get the upper hand – then I stack the scenes together in the order that makes sense (THIS because of THIS because of THAT) and throw the conclusion in at the end, which, by the way, would have been one of the action ideas because I never launch into writing a book without that all important conclusion to aim for – and BAM, we have the makings of a book!
That “BAM”, be warned, might take anywhere from weeks to months (or even years), depending on available time and the size and complexity of the story.
Then the work kicks in as the journey is composed, the getting from (or to) action scene A to action scene B to action scene C, etc., down through the line.
Of course, other action scenes pop up along the way, making the whole thing much more fun. Characterization develops, too, adding surprises. Additional characters are brought in as necessary for both background and foreground, adding depth and detail.
Research, naturally, must take place. Someone somewhere is going to specialize. Even if you have mundane characters doing mundane things (ordinary folk in an ordinary story), you want the psychology to be right and you’re going to need variation if you don’t want all your characters to look and sound the same.
Other research is necessary, also, not just where someone specializes. The How, the What, the Who of any area you, as a person, are unfamiliar with must be examined. You can’t just make stuff up about weapons handling, for instance, or factory safety, or emergency procedures for evacuating a hospital (just plucking stuff out of the air here – of these examples, I only wrote about the weapons handling – but you see what I’m saying).
By this stage, a writer wants to know each individual character quite well so that they can step forward and automatically be themselves whenever called upon. This takes some of the effort out of it – you don’t have to THINK about how this or that character will respond, you KNOW. All you have to do is bring them forward and watch it happen.