According to Wikipedia, a character flaw (sometimes called a fatal flaw) is as follows:
Character flaw – Wikipediaen.wikipedia.org › wiki › Character flaw
In the creation and criticism of fictional works, a character flaw or heroic flaw is a bias, limitation, imperfection, problem, personality disorders, vices, phobia, prejudice, or deficiency present in a character who may be otherwise very functional.
How do you choose a character flaw for your main character? That is up to you. I centered it around the story. Once you choose a flaw, how do you use it within the story? Is it just there throughout and that’s all and the character goes about their business within the story and never learns anything from circumstances or their actions, etc? If that’s the case, do we even have a story? No. Not really.
The protagonist in your story is supposed to learn along the way. Their character flaw inhibits this at first, but gradually he/she comes to realize they need to change in order for their mission/goal/quest to succeed. By the end of the book they will have overcome this flaw.
Let’s say you have a main character named Molly, who’s in high school. Her flaw is that she’s timid/shy. She doesn’t like confrontation or violence and prefers to avoid it at all costs. She doesn’t like to hurt the feelings of others, so she avoids situations where she might have to speak up and maybe make people angry. Therefore, she doesn’t speak up for herself either. So, how does she learn to overcome this and become stronger inside? How does she get others to stop picking on her? How does she speak up for the truth and defend her friend Johnny?
In the above example, you have to get Molly from point A to point B to point C to point D; point D being the end of the story where she finally speaks up and gets over her shyness. Getting from one point to the next will involve a series of events called trials and errors that will serve as learning opportunities for her grow, but she can’t do this without these trials and errors.