I was at one event where I was out in the lobby area before the author's reading and he came by. I was nervy enough to ask him if he'd mind autographing the two books of his I'd brought, fully expecting him to just jot something and then head off, but he stuck around and chatted with me for about five minutes. It was really cool, but not something I expected at all.
On the opposite of the spectrum, I've been at cons and run into an author in the general hubub and asked for an autograph on a book and got the impression they'd rather be anywhere else. One in particular didn't say a single word to me, just took the book, signed it, handed it back and walked off without having even made eye contact. It left me feeling guilty for having asked.
I think you need to gauge the person and the situation and act accordingly.