You have probably heard about the importance of knowing your fictional characters so well that you know what he/she had for breakfast. Readers don’t need to know this, but the writer does.
You don’t need to include everything you know about your characters in your story, but as the writer/creator, you need to know a huge amount of information about the people (and animals) who populate your story.
The challenge is to create memorable characters rather than one-dimensional characters. Your fictional characters are like actors in a scene.
Some fictional characters seem shallow while others seem richer. The difference could be that the writer knows the characters/actors so well, that the dialogue and the details fit the character.
Your fictional actor may want to step out of character and exhibit new behavior. This is fine, as long as it’s credible. Your job as writer is to drop convincing clues so when the character does an about face, the reader believes it. You can still have twists and turns that are surprising for the reader, but everything needs to be consistent with what the character would or could do.
Is your character a loving husband who shows his affection with gentle actions towards his wife? If yes, then it would be out character for him to leave her stranded at a party. There would need to be a reason for his out-of-character behavior. Maybe he found out she isn’t who he thought she was.
If your character shuffles in worn-out bedroom slippers, listens to the radio from 4:30 pm to 6 pm in her favorite armchair while knitting, then goes to bed at 7:30 pm, it would be strange for her to dress up in Spanx and a tight red dress to go bar hopping. She could do this, but you would have to set up the scene so it’s believable.
If you portray your characters as authentic, then when your characters drive off a cliff in a convertible, the reader believes they would really do this. Yes, I’m thinking about Thelma and Louise.
Want to practice?
Write three scenes.
Show your character in an ordinary scene . . . something they usually do, their routine, their habits.
Write a scene with details about what might make that character go over the edge, a “last-straw” type of thing, a friend or a relative did something one too many times. Or the character receives news that spins his/her life in a new direction.
Write the final scene showing the character exhibiting new behavior.