If it's a comedy - now, this is not a hard-and-fast rule, but I've kind of found it to be true. If it's a comedy, you get your job description and your first name, like in "Wild Hogs" I played Sheriff Charlie. I played Ringmaster Bob, a lot of roles in which you get the job and your name.
If you are playing a serious role, you get the job description and your last name, Detective McClaren, Agent Jones. Now, then there's a level below that, in which you get no name, you get no name. You just get your - sometimes your job description - homeless man, man on train, man with a limp. You get these. These are usually not very good roles.
Yup, that's about right.