Bram, well said and some of the best advice that a founder can take. I can attest to it as a startup guy who spent a lot of time talking with the prospective users of my platform before I committed to building the platform I was researching.
I have an additional thought for this, and that is, talk with everyone who will touch, or feel, your app/service/product and don’t leave anyone out. Test all your assumptions about your idea. It’s not enough to get responses to the idea or UI. Ask about payment the stream and process. If you are developing a B2B service/app/platform, talk with how they are invoiced and what platforms they use currently. Ask your users how your proposal changes their existing processes and if there is a difference you are asking them to accept, bridge the gap, if necessary, to your ultimate goal, if you encounter strong resistance. It will pay off in quicker user acceptance and more reference users for you to incorporate into your marketing.
The more potential users you can interview, the fewer significant changes you will make when you roll out your app and, the sooner you can reach “hockey stick” adoption rates in your marketplace.