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

The most interesting things you learn in an interviews come from the: 'interesting', 'tell me more'

The crucial people to get your product started for the first 6 months are not who will be using it 3 years later.

The other mistake I think people make is talking to who's available rather than talking to who they need to talk to.

Users think they know what they want, but you get the horseless carriage effect where you're getting asked for a faster horse.

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

I wish I could tell you the recipe for figuring out who the target user is for your product and who your users should be, but... there isn't a recipe. It comes down to think really hard and use your judgement to figure out who you're really building this for.