I recently interviewed with 5 different people for a new position at the same company I work for now and one of the questions they asked me was how do I define success.
I told them I consider both tangible and intangible factors. For example, in my current position (help desk) I look at things like my average call times, my first call closure percentage, the number of calls I take and the number of cases I resolve compared to my peers. The intangible things I consider are whether I feel like I've accomplished something. I may have spent an hour on the phone with a customer but got the issue resolved. If I didn't, what would have been the impact? Was it worth an hour of my time where I could have resolved 6-8 other problems to get this one problem fixed? Even if it was worth my time, could I have used my time more effectively? While I was on hold waiting for the customer to trace a cable, could I have processed one of the many reports that are generated throughout the day, could I have taken a couple minutes to do proactive maintenance on a server?
I think they liked that answer.
In your case, you might want to try to show more confidence. It sounds cliche, but you're interviewing them as much as they're interviewing you. Each time, I walk in there with the mindset that they've already offered me the job, and I'm getting information from them to see if I actually want it.
Before this set of 5 interviews I just went to, I've had three others in my life. All three resulted in job offers, two of which I took, one which I rejected. We'll see this coming week if I'll be 4/4.