So, you have your two ideas. The high concept, and the aspect of the story that is going to make the first reader sit up from her usual slush reader slump and actually start caring about what’s going on the page. You don’t need to know the exact beginning until you get to the end and rewrite it, but as far as you know the story as it is, you start at the Carmichael startling point.
Leslie Carmichael was a writer friend, lost to cancer, and an amazing person, if you pardoned the dreadful puns. But calling the beginning of the story the startling point was brilliant. It’s the moment before your character’s path changes dramatically and your character who was going about on their daily life suddenly can do something much bigger. When I get to this point, I usually have no frelling idea as to what the “thing” is, but I lay the pipework in the story for there to be the thing in the story that is worth your character pushing on so that they will continue with the problem longer than anyone who is just being paid, or coerced or just going along for the lulz would pull up their stakes and go home.
And for that you need a reason as to why this character is the one who is called. They have to want something more than anything else. In the beginning of the book, what the character thinks they want doesn’t have to be what they will want at the end, and actually it’s better if it isn’t. There are going to be barriers to the character getting that thing (and at least one of them has to be internal) and the first half of the novel is your character finding out what the problem and the need is.
The worst thing you can do is have the big problem happen before the book even begins and have your character as a clean up crew going along and sweeping up what has already happened. I know it can feel like it’s impossible to change the timeline in your story even before you write a word. What happens just seems to need to have happened. But if the most important thing happened before the book you want to write is going on, you’ve probably got the wrong startling point.
In media res is like Odysseus’s sirens, seductive but will kill your momentum, and you have to strap yourself to the mast to avoid it when you’re pantsing. The forward motion of your plot is what pushes the story forward. If you start in the middle you have to tell what’s happened before, and you can kiss away any of the tension or momentum you’ve built up. It can be done, but if you take that moment that makes or breaks the character that most people start their book with and plant it at the end of chapter three when your character and your reader get to the moment where everything falls apart but this time the audience is up to speed as to what is at stake, what’s happening, why it’s happening, what’s going to happen if everything goes wrong, who are these people, and why should they be cared about will all have been answered if instead of starting with the ah, shit explosion, the book starts with the first warning tremor.
Done right, it sets up the rest of the story for success. Now that everything has gone wrong (but in a way the main character can feel as though they can make it right) it’s just a dance of successes and failures to the two thirds point, where everything goes wrong in a way the character doesn’t feel it can be made right. That’s about it for today. Writing is all about controlling disasters no matter what your genre in a way that airplanes descending down to the runway is just a controlled fall.