I tend to use contradictions and quirks a lot in my writing because I feel it adds a little something extra to the characters. Although to tell you the truth, I don't overthink it either because it comes along naturally. Each character tends to have unique personalities that encompass fascinating contradictions.
As an example, my character Diego Silva is very dapper and sophisticated. So it's surprising that his weapon of choice is a baseball bat. If you read my Hernandez series, you will see Diego bring out the baseball bat on more than one occasion.
Another example would be Paige Noël-Hernandez, who is considered one of the best assassins in the world...who meditates. She also used to have a self-help site online. Contradictions like this are neat. It shows contrast in the characters.
Contradictions make characters seem more realistic, fun, and interesting. And let's face it, we all have the contradictions that make us unique. What are yours?