I feel quite comfortable about talking to people I don't know at all and probably ain't ever going to have to see again.
However, if there are people I might have to get along with but don't know, then I get kind of nervous. Who am I talking about? Well, people in my semester I have nothing to do with, for example. Or new people I know I'll have to work with for a while.
It's kind of weird, but...
People I know closely and complete strangers I feel comfortable around.
People I know barely but might have to keep dealing with, I feel uncomfortable around.