An early structure pitfall
I love structure and find it an incredibly useful tool. But it comes with its own built-in pitfalls and here is one of them: if you plot your story to a specific structure from the very beginning, there is a danger that you will funnel your characters down unbelievable paths in order to get them to hit certain beats.
If your story target is too clear even before you have a sense of what is happening, what the characters are doing and why, you start to herd your characters in ways that restrict the storytelling. And in the process, often making the characters behave in ways they shouldn’t.
It is an all too common story problem that characters do things to serve the needs of the writer rather than serving their own needs. It happens so easily. I know because I have been guilty of it myself. I have a great idea for a scene but, to get there, I have to push characters in ways that don’t quite fit who they are or what they should want. And because it is all too easy, you have to be very wary of anything that invites that risk into your work methods.
Nailing down your structure before having a sense of your characters and story can do just that.
My advice is: know your premise. Know your characters. Know your starting point. Then let your characters take you where they should go. Let your mind wander in and out of the scenes, each character playing it the way they naturally should without you worrying about the end point of that scene. Make notes on all of that stuff until you have enough to build a story. And THEN sort out your structure.
That way, your characters are living, acting and reacting. It isn’t just an exercise in herding.