Characters are what has happened
Any writer worth their salt appreciates that for characters to be convincing they have to have had a life before they walk onto the page. They have to be motivated, they have to have a reason to do what they do. If your characters are behaving unconvincingly or refuse to do what you intended it’s probably got nothing to do with them being under-developed or not defined deeply enough - it’s because you haven’t given them any reason to behave that way.
Don’t invent some innate trait or neurosis to justify some unlikely behaviour - think of a good reason, an event, that would make them behave like that; make it a reason that people can understand, a reason they can relate to - it is this quality that makes characters convincing. If the reader is sympathising with the character, feeling anger, happiness, fear, or sadness because of the events that have happened to them, they are projecting themselves into that character. Your need to describe the character’s hairstyle, what their handwriting is like, what side of the bed they get out of is unnecessary, because your audience is becoming one with her, they understand her. They understand her because of the things that have happened to her.
A character with a single but compelling motivation will be more convincing than a complex character without one. If you’re struggling with a character, think of the external events that have made that character who they are.
Characters are what is happening
There are character defining events that have happened in the past, and there are character defining events that are happening right now. It’s the difference between the back-story and the front-story. The events that have happened in the past define your character at the start of your story. The events that happen in the story effect change in your character - they signify what your character is about to become or remain - either a reaffirmation of the character or by pushing them through an arc.
Characters are how they react
Characters are not what they say they are. Characters are not what they talk about. Characters are what they do. The real test of a person is how they act in a crisis. Think of all those people at work you think you like until the shit hits the fan - then you really know who you can trust. That’s why every hero needs a crisis. If life was ticking along without any problems to solve, without any need for bravery or for someone to step up, we wouldn’t need any heroes. It doesn’t have to be a huge thing - how does your character react when they burn their toast or when their kid disappoints them in public? This is when you find the measure of people - how they react to a situation when it’s easy to be bad (when no one is looking), and how they react to a situation when it’s hard to be good (have-a-go hero).
Sometimes you don’t really know your character until you start writing - it’s only when you see them in action can you truly get to know them - just like real people.
Why story beats character every time
Ten questions to ask your characters
Five ways to know your characters before you even meet them