I think interviewing people is a good idea. The fact of the matter is at the "idea" stage you don't have enough information to present to people to really give them a feel for if your specific product is viable. But by interviewing a fair number of people, you can understand what their legitimate needs are and what they're willing to pay for it.

Of course it's not foolproof, but I think it will actually be better than the approach given in this blog.