Interviews don't always go so well for me

I noticed I subtly make mistakes during interviews that the interviewer picks up on. In my last interview I had at panera bread(which also has great coffee) I made sure I had great body language and didn't slump back. I leaned in and I would listen intensely and would "talk with my hands". When before I would just look like I was bored. I still don't know what I'm supposed to ask when they ask "do you have any questions?" I'm tempted to say "when can I start?" and see if that will close the deal.  I had an interview one time at a autoparts store and you know that feeling that things are going bad? I could tell that interview went south real quick when I lost my composure during some of the autoparts questions. I started stuttering and saying umm a lot. I was really close to saying "this isn't going well is it? thank you for your time" but I sticked with it even though I had no chance of a call back.