Some years ago, I either read or heard a statement about characters in a story that went like this: Every character in your story is on their own journey. The main character certainly is, but so is everyone else - without that, they are either one-dimensional props, or are left hanging in space. It was one of those revelatory moments for me - I had never thought about it like that before (we all have long periods of being dumb about things to do with writing - it's reading and listening and thinking that gets us past them!). Up until then, my focus had been the main character - what he/she wanted, how he/she did or didn't achieve their dream or goal.
That, of course, is the core of the story. Fred has a dream to climb Mt Everest and will do anything to achieve his dream, even beg money off his hated mother and agree to take the bank manager with him to Nepal (I use Fred in class as my running example of story/plot/motivation/complication - one day Fred will make it). But when I realised that giving every significant character in my story a journey, a desire, a goal, is what enriches and complicates the plot, and makes a short story into a possible novel, that's when I think I finally started to "get" what I needed to write something more than 10,000 words.
For others, it may be different. I've seen the light go on for students when we talk in depth about character motivation, and I keep asking them "why, why, why" about what their characters do and how they react. I've also seen it happen when we talk about setting and description and link them to characterisation and point of view. Everyone has different things they discover about how to write better, and it often results in a big leap forward in the quality of what they write.
So for me, it was this thing about a character journey, combined with the question: What is the highest point in the story for this character? That goes along with the other question, funnily enough, which is: Does this character have a high point in the story? If not, why not? If we bring it back to the journey (the hero's journey, if you like) it's the climax, the supreme ordeal, the moment of greatest change or challenge. It's the turning point for that character. It is where we finally start to see whether they will live or die (sometimes literally).
Another mistake I used to make was to rush that highest moment in the journey. When it's the main character, that point means you are close to the end of the novel. Wow, only twenty more pages to go, I think. Even less if I get a move on and get through it faster. Big mistake. Rushing the climax takes away the tension, the full exploration of what it means, and the potential of the resolution. If you read articles about dramatic scenes, you'll know that the intensity of drama in a scene usually determines how long that scene will be. The most intensely dramatic scene should be your climax, so there's a good reason why it may well be your longest scene. Not, as I used to write it, the shortest.
It's helped me a lot to imagine each character's journey in my stories. It especially helps for the antagonist - instead of being a cardboard villain, she/he has their own story, their own journey, their own desires and dreams. OK, that dream might be to burn down the school, but for that character, it's believable if we understand why. We don't have to agree, but we do want to see what motivates that character to walk that particular road to the end, no matter what.