What is character development?
Your character was brought into the world as a blank slate. Whether they hatched from an egg, came from the mother's womb, or popped out of dad's head, their personality and demeanor was shaped through nature and nurture. The key to character development is to create a realistic reaction to an event in your character's life. For instance, maybe your character was a flirt and was bubbly. After being in an abusive relationship, she changed. She became introverted and scared, not meeting anyone's eyes nor speaking up with a clear voice.
Or perhaps your character was a bully. One day the tables turn and they become a victim and suddenly they open up their eyes to their own behaviour, and become a better person for it. They begin to treat others with dignity and respect, maybe even apologize to those who they hurt.
What makes up character development?
Personality traits, nature and demeanor, appearance, relationships, history, strengths, weaknesses are among the things that shape who your character is. Think about yourself. Who, what, and why are you the way you are today? Do you think you will be the same tomorrow if ___________ happens? Your character may be the complete opposite of you, and may react to a certain situation differently than you would. That's perfectly okay. But to keep it as realistic as possible (notice that I've used realistic several times; realism is very important!), you should compare yourself to your character. Suppose you were like your character, how would you react in their shoes? Or if they are like you, how would you react in a situation?
Remember that speech, demeanor, appearance, and movement are things that your readers will picture and hear in their mind when they see your character. Always add in a description of their thoughts, opinions, nature, ambitions, flaws to add realism, even if the other participants' characters don't know themselves!