Let me clarify:
The character need not be a minor character. Usually, stereotypical characters of stories are the flat minor characters. Pertaining your assignment, you can write a story where the main character is flat.
In order for Bill to be flat is show he has no depth and does not grow. Usually this would include that he makes [life altering] mistakes but does not learn from them. For instance, Bill nearly gets into accidents because the game is more important to him than driving safe.
To make him dynamic, portray him in the same way, but describe him as being shocked after a near accident- make him realize that it is just a game and he has to drive safe. ... Or something.
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