I'm writing the outline for my first novel, which has three main characters. I'm very happy with their names, however I recently noticed that I was naturally referring to the first two by first name, but to the third one by his last name. I think it is because 1) I like that last name, which is a very strong and rare one, and 2) it is the name of someone I've known for long and who I've always addressed as "Mr Last-Name."
The story is a thriller involving two members of the mafia and one old tired french cop, if it has any relevance to the question.