I build things iteratively all the time, without anyone seeing most of it. I move things up for review and QA after I'm satisfied that I've done the things that need doing, and done them well enough for purpose. Customers aren't the only opinion on what's good or bad. They're more like the final and most important.

Now that process is much, much faster.

Presumably you have at least a partial spec, and some domain knowledge. If your invoices doesn't show amounts or a recipient address, for example, you don't need a user to tell you that is an error.