If I hang out with a famous person, I have to temporarily forget that they are famous, or I get all weirded out.
I find it helps (in the case of authors) to have some sort of detail-oriented question about a character or the setting, that treats the thing as having objective reality. I have yet to meet an author who didn't like to explain some of the things they know about their world that didn't quite make it onto the page.
Musicians, well. I'm with you there. It helps, if someone is signing the HUGE STACK of compilation CDs having ONE TRACK EACH by their band on 'em that I've dorktastically brought to be signed, if we can sort of ignore that and talk about Doc Martens or Rocky Horror or other unrelated topics. (totally true example, btw.)