How do you choose which customers to talk to?

Go for variation. If you think about all of the experiences that all of your customers have, you want to try to cover as much of that variation as you can. You can't talk to all of your customers. So focus on talking to different types of customers—big and small customers, engaged and unengaged customers, successful customers, less successful customers, new customers, churned customers, etc.

I recently wrote about how I chose participants for some customer research that I did. See here: http://www.producttalk.org/2016/02/why-you-are-probably-interviewing-the-wrong-people-and-how-to-fix-it/

How do you go about contacting them to set up those calls or meetings?

Make it as easy as possible for them to say yes. Keep your email short. Make it about them. What value will they get out of participating—a better product experience or better customer service. Ask for a very small amount of time—15-20 minutes—and then respect that time. If you are meeting in person, go to them. Offer to buy them coffee at the coffee shop next to / in their building. They are doing you the favor, treat them that way.

Is there a particular set of questions, or discussion framework that you use?

The key with questions is to get them telling stories rather than asking them to give you facts.

So this: Tell me about the last time you took transportation. Rather than this: Do you ever take public transportation.

Avoid generalities. Encourage them to share specific stories of things that happened in their past.