I've been designing and coding web sites for nearly 14 years now.
Anyone in a similar position can agree that comes with a reputation amongst friends and acquaintances as the guy who can build your "amazing idea called mygooglebook," that will obviously be bigger than the beatles, if they were a web site.
This is about the time I usually try to take a huge dump on their idea.
There is an inherit value of doing so:
- If they really believe in it, what I say should only motivate them to prove me wrong.
- If I'm right I'm going to save them a lot of time.
- If they can't convince me I'm wrong, how will they ever get customers?
- I'm a busy guy, I don't want to be pitched non-stop. If this stops someone from repeatedly pitching me shit ideas, then I've saved us both some time.
Lets say I do decide their project is enticing enough that I would consider putting in my time.
This is when customer development becomes your best friend. At this point its easy - you want to have shared (not equal) effort and founder value if you're working together. You know you can do your job well, but you want to mitigate your risk entering any kind of joint venture (with a friend, acquaintance or otherwise).
So now you must require your "idea/business man" to develop a customer base, that will be ready to buy on launch. You need this absolutely guaranteed - by the customers, maybe a form of early discounted payment, letter of intent, or early deposit.
If your business guy can make that happen before you ever touch the keyboard, you're doing it right - and they're the right partner to have.
So, unless you like to "do it -all- yourself" - get dumpin.