No matter how complicated and realistic your characters are, unless they undergo some sort of change within your writing, they won't resonate with your readers. The evolution of a character can be heartbreaking, hilarious, or both. Most importantly, the journey of identity your characters go through must be compelling.
There are plenty of changes your characters can experience. Each person has their own unique story, but as you write, you'll discover a few recurring themes in character arcs.
Everything begins with the ghost. Your characters have a person, experience, or other event in their past that haunts them. It's something that follows them, and eventually they'll have to confront it.
All characters are driven by a goal. These goals can be emotional, like love, envy, or fear. They can also be material, like wealth. It's critical to know each character's goal as you write. Motivation is the reason a character stays with a person or abandons them, the reason they power through a bad situation or gives up. Know how strong your character's goal is, and why it's their goal.
Every character believes in a lie that keeps them from their highest potential. A character's goal is shaped by their lie. This can be their perception of the world, or a false understanding, or a superficial assessment. Even a literal lie works. Whatever you chose for your character, the lie they believe must hold them back until they encounter their truth.
A character's truth is the final conclusion a character comes to. Your character's truth is not necessarily a positive one, but it is what your character now believes after the events of their story. The truth can be the moral of the story, or a cautionary tale. It all depends on your specific character.
These four elements are the core components of a character arc. This pattern may not be obvious at first—the structure of an arc changes wildly depending on which one you're writing—but if you look close enough, you'll find these are universal factors of a writer's craft.