How to find people to talk to

The only way I know how to understand how different users think about your product is to speak to them.
You don’t need to speak to lots of people, nor do you need to speak to them all at once. Two or three people a week is all you need.
Recent customer success wins are always a good place to start. You don’t need a complicated reason to reach back out to these people. Explain that you want to improve your product and that you’d like to understand how they use it. Clarifying that it’s a short call helps.
Conversely, when people reach out to the customer success team, you can close out a successful interaction by asking if they’d be open to schedule a quick conversation with the product team.
Eventually, you will need to be able to pick who you talk to. 
If you look at last month’s user activity and plot out the number of times each person performs your core action you’ll end up with a bar chart of how many people performed how many actions. Those who did it once or twice will be on one end and people who did it the most on the other end. Filter out anyone who signed up less than a month ago and then reach out to 10 to 20 people in the top and bottom 5%.
Another approach I’ve had success with is doing in-product surveys. NPS scores and those satisfactions ratings that show up in the corner of people’s screens. You can end a quick survey like this with a request to schedule a call.
If you have other approaches to recruitment that have worked for you please let me know. I’ve always found recruitment to be the hardest part of doing customer interviews.

In terms of doing the actual interview, I’ve squeezed everything I know into this post.