Ask HN: The best way of Costumer Development

Hello Guys, I would like to know what you think are the best ways of doing the Costumer Development. Me and my co-founders we are going to do in 2 ways. The first one is a form, with questions about the product. This is good, I think. But do you guys know something better or more "fun"? The second way is doing a just a picture of what our service would be like, and ask people to use it. Anyway, I would like to know if you, guys, have something better to do this valuation. Link and blog posts are welcome. thank you.

Eduardo Rocha