Final answer:
The author's use of colloquial language.
Step-by-step explanation:
The line, "'That ain't my style', said Casey" demonstrates the author's use of colloquial language. Colloquial language refers to informal, everyday speech that is commonly used among a particular group of people. In this case, Casey is speaking in a casual, non-standard manner with the use of the contraction "ain't" instead of the standard contraction "isn't" or "am not". This helps to convey Casey's character and personality.