So I think that there are a dozen “character development” sheets available on the web. As writers we all have them. We all look for new ways to put the characters together and not end up with crap.
The best way I know of is to decide what your story is about. The core of it. Then you “interview ” your character. Ask questions based upon the plot. You should already know the basic stuff. You need to ask about why they are in your story. What is their role? What does the story mean to them? How does the course of the story affect them? While yes you can look into the character, learning about their favorite music, books, and such… Unless you know the reason why they are in your story it will be fluff and not help you write.
You should understand how they are interacting within the story. With other characters, with the environment, and then with themselves. Is the character self-destructive? Why? You should always know the why of any behavior you give the character. The basic psychology behind their behavior.
Now that sounds scary… I know. Still cause and effect gives you a lot of details. And while the reader may not always see what caused the character to behave like they are, you should.
So I will give you an example. In Serena’s Rust, Gore, and the Junkyard Zombie… Chris is the sort to try to do everything by himself, and tries to be seen as an upstanding citizen. Now we do learn why in the story if we pay attention. His father was the town drunk and his mother raised him and his sisters by herself. Chris grew up basically as the man of his house and bullied by townsfolk because of his father. People who have that background can show it in multiple ways. Chris showed that he could be better than his father, but did not trust others to help him.
So what is the why for your character? And how does it fit in your story?