I like to ask if "this" solves a big problem for them, if it makes their life easier. Is it something they feel compelled to use once they have started using. It can get dicey when you ask too many hyper focused questions as you can get people who are way down in the weeds on certain features but then again you don't want to miss out on a pearl of an idea. It truly is a balancing act.
It really depends how long I talk to each person, some people you can tell pretty early on are going to be very helpful and I try my best to get them to talk to me as long as I can. One thing I always try to do is record those conversations, with permission, in order to come back to later and take better notes on what was said.
Watching people use your app can be illuminating too but it can also be a bit misleading because people won't always act/interact with your site/app the way they normally would because they are being watched.
Good luck with customer development, it's fun/rewarding/daunting.
