What others have said. A photo op, which is usually pretty quick, enough time to say hi and that's it. They also come on stage, talk for a bit and tell a few stories from behind the scenes, then they let you ask some questions. Don't ask stupid questions though. I heard one guy ask what it was like to go through the transporter. Later there's the autograph time, again, just enough time to say hi and that's about it. The cons I've been to usually have an auction as well.