The most common mistakes are showing people your product- don't show them your product, it's sort of like telling them bout a feature.

You want to learn about what's already in their heads. You want to avoid putting things there.

The one question you can't ask is, "Is this feature actually good or not?"

Most startups are not just built for the person who is using them. When you do that, every now and then you get really lucky and... are representative of some huge class of people who all want the same thing you do... but very often that just turns into a side project that doesn't go anywhere.

The instant they say something you don't expect or already know, you should drop into detective mode.

Sales is this cure all for this problem. Get people to give you their credit card and I guarantee you they're actually interested.

Recording interviews is like magic. a) It stops you from taking notes in the middle and b) you can play that recording for people.