Nobody’s going to steal your idea

Only when the idea precedes the exploration of the opportunity space.

Have an idea -> Build a prototype -> Show it around -> Close some sales.

Here the idea is related to the market opportunity. It triggered a search for the opportunity when someone was interested or puzzled enough to do the work. Few will invest in an idea unless there's some kind of "sugar on top" that reveals really good potential for a market opportunity. Many will invest in a reasonably tested market opportunity.

When are an idea and a market opportunity not connected? When the idea does not precede the market opportunity.

I'll give you a couple of concrete examples from my own life.

A friend of mine came over one day many years ago and said that his contact at one of the local film studios told him that they were looking for a carpenter that would make them 100 sets of this specially designed piece of furniture for an expansion project. My friend knew that I, when not coding or designing products, liked to, among other things, do woodworking projects. I also had a pretty niche setup in my garage at the time.

So, here's an opportunity presented to me which had not originated with an idea.

What did we do? We spent that weekend making an unsolicited sample of the piece the studio needed. He delivered it on Monday to his contact along with pricing. By Wednesday we got the deal. By Saturday I had a pile of furniture-grade plywood in front of my house that was eight feet tall. We setup an assembly line and spent a week (with some help) cranking out the pieces. My neighbors were puzzled and probably somewhat concerned. A couple of them came over to help for a few bucks. Delivered on time and walked away with $60K profit. Sweet.

Any able carpenter would have taken that opportunity and run with it. We just ran faster and put a solution in front of the customer as painlessly and well-executed as possible.

Another time I had someone call me to tell me that there was a company in need of a box built to translate a communications protocol in real time. RS422 in and out. Status display. Power button. The conversations went something like this:

    Me:  "How many?"
    Him: "Probably 25 per month"
    
    Me:  "How much are they willing to pay?"
    Him: "They've been trying for months but it's a bunch of
          Optics PhD's and this isn't their thing.  
          They are desperate for a solution".  
    
    Me:  "Why?"
    Him: "They have sold a bunch of these units at about $500K
          a piece and this has been one of the only reasons
          they can't deliver."

    Me:  "I can meet with them tomorrow AM"

I built it in a weekend: 1 RU enclosure. Off the shelf 8051 board. Small custom board with RS422 drivers and receivers. LCD display. Power supply. Connectors. I don't think the total cost exceeded $150. It would have been a sweet ride except that about six months later they got acquired by a large multinational and, effectively, shut down.

Again, a market opportunity that did not come from an idea.

Now, what if I said? "Let's build an RS422 protocol converter box and sell it for $3,000". And I did this in complete isolation of having identified a market need? Well, it'd be a flop. Sometimes fining opportunities is like finding a needle in a haystack. Someone almost has to sit on it for you to become aware it even exists.

It's hard to come up with an idea without at least some intuitive sense of what kind of need/desire its fulfilling. I think people deceive themselves here because they have a naturally inflated sense of how great their ideas are.

To use your phrasing: people think their idea is great enough to require very little testing to become an opportunity - their intuitive sense of how great their idea is is enough to begin immediately calling it an opportunity.

Whenever I hear a common refrain in an industry that something doesn't happen, I know immediately that it absolutely does happen, but that acknowledging it doesn't fit with the existing culture for some reason.

Napolean Hill (Think and Grow Rich): Ideas without action are impotent

Thomas Edison: Genius is one percent inspiration, ninety-nine percent perspiration (he became a litigious bastard anyway)

If you're in business to make money, almost without exception it's going to be all about making and keeping people happy. That takes ongoing work, it only starts with an idea.