Maybe this is implicit in calling something a "startup" rather than a "side project", but on both sides there needs to be a common agreement on how much time you put into the project, what your vision for the growth of the company is, whether you have other sources of income (and work) at the same time, etc.
The nasty variation on this that I ran into was something that started as a side project where two of us had full-time jobs and one guy did contract work. A couple months later the other guy got laid off and started putting most of his time into the side project, which then evolved into a startup in his mind. Much acrimony followed regarding how much time each person was or should be putting into the project, whether things were "fair", etc.
I won't claim to have any solutions, except that in the future I'll be a lot more careful about asking people what their intended time commitment is, and where they want the project to go. I'd also insist on written agreements from the start.
In the case of my project, there was about 6 months of anger, finger-pointing, and general failure to make meaningful progress because all our time was consumed with debates over how the project should operate. Eventually, I realized that the project was a major source of unhappiness in my life and I announced my intention to leave the project. We reached an agreement for the other two to buy out my interest in the project, and they have since made it their full-time job and are attempting to turn it into a real business, working with a local startup incubator.
The nasty variation on this that I ran into was something that started as a side project where two of us had full-time jobs and one guy did contract work. A couple months later the other guy got laid off and started putting most of his time into the side project, which then evolved into a startup in his mind. Much acrimony followed regarding how much time each person was or should be putting into the project, whether things were "fair", etc.