You're saying no ninety-seven times out of a hundred, and most founders find they have to make a very conscious effort to do this.
You really want to know your cofounders for a while, ideally years.
No matter what you choose, build stuff and be around smart people.
The other piece besides focus for execution is intensity. Startups only work at a fairly intense level.
You'll get more support on a hard, important, project than a derivative one.
It's easy to move fast or be obsessed with quality, but the trick is you have to do both at a startup.