Share results (financial and key metrics) with the company every month.
A lot of people treat choosing their cofounder with even less importance than they put on hiring. Don't do this.
A related advantage of mission oriented ideas, is that you yourself will be dedicated to them.
Cofounder relationships are among the most important in the entire company.
It really is true that you become an average of the people you spend the most time with.
Remember that the idea will expand, and become more ambitious as you go.