1. Motivation: give your characters reasons for acting the way they do
If you read reviews on Amazon, you’ll often find readers complaining about characters’ motivations. “Then suddenly, for no reason…” “Just because it had to happen for the story…” etc.
In real life, people do weird things. Your story people can do weird things too, but you (and readers) must be clear on why your characters do what they do. This means that if you know that something is going to happen at the mid-point, you need to plant the seeds of that earlier.
You won’t always be able to do this in your first draft of course. However, it must be done. Every character needs clear motivation for acting the way he does.