I've been doing lots of customer interviews lately, and I'm absolutely loving it. I think this bit of advice from the article is dead wrong "know exactly who you want to talk to and what you want to know".
I've got a product that is growing nicely and that users love, but we haven't been able to monetize it yet. Lots of other comments point to things like Ford's (misattributed) "they'd say I want a faster horse".
Don't ask people directly what they want, find out what they are doing, what they need to do, and you'll start to find a pattern emerge.
The reason I feel you can't know what you want to find out or discuss is that you can miss exactly what the customer is telling you.
In my experience, I will often ask pointed questions about what more we can do, and I get blank stares and uses just say "it's great" and give me requests for minor improvements. When I probe them about other parts of their business, they open up about what they are doing in other segments which to them are completely different than what we do, but when I point out the similarity, and what we could do in these other areas to help them out, they perk right up and come up with a bunch of good ideas that they are happy to pay for.
So be flexible, and look for patterns more than answers. I'd also suggest, if you're in the same place as I am looking to monetize an existing product that most see as free, ask where they are spending money and see if there is opportunity for those funds to be re-allocated to you.