The questions I wrote down as a template in interviews are a good base, but that won't cover everything. You'll need to dive deeper, and I wanted to document an example from my personal experience that might give you solid expectations for how to dive deeper when the opportunity presents itself.
I interviewed thrift stores to see if I could create a product for that type of business. My first interview ended up being two hours long.
The manager of the store gave me a full tour of the store and showed me every process they went through to get clothes from donations to customers.
At each part of the store, I would ask how much time each process took, how much it cost, who handles the process, what tools they use to make that process more efficient, how each of those tools work, and how well on a scale of 1-10 that tool works currently.
I did this for:
You're searching for problems. If your interviewee complains about something, you ask why it's annoying, how they've tried to solve it, why none of their solutions are working, what they wish they had to solve the problem and anything else you can think of.
If you can find a common problem between a bunch of similar customers, and you can solve it, then you'll get paid. Just focus on the pain points your interviewee is experiencing.
[The above example is for a business, it will be more casual and directly related to a habit, personality trait, hobby, interest, etc. of your interviewee if you're doing consumer-focused interviews.]
Lastly, if you're serious about interviewing a lot of people for a serious product build, start a newsletter of people you’ve met with. Ask higher-level professionals or experts to join a product advisory board you can bounce ideas off of.
Keep in touch with everyone you talk to, they're gonna be your first customers if you solve a real problem of theirs.