It tends to be a very long process, and often involves a lot of "trial balloons." I just went through that, in the last couple of years. The project we're realizing, looks absolutely nothing at all like what the CEO originally dreamed up, but everyone that has seen it, loves it.
The trick is to not start off by saying "It'll never work." That slams doors shut, right away.
It's more like, "OK, so let's walk through what we'll need to do, to make it work."
That will often result in changes being made, by the "idea person," as the plans are laid. We will also try to create test harnesses and prototypes. These often end up, with the idea person going "Hmm...it seemed like a good idea, but it doesn't work the way I wanted."
It's slow and painful, but works.