Jessica Alter, cofounder and CEO of Founder Dating, recently authored a guest post on Steve Blank's blog titled "How We Fight -- Cofounders in Love and War." Her thesis is that one of the most important criteria for selecting a cofounder is knowing how you fight with them -- how do you resolve differences? When there are disagreements, do you resolve them quickly and move on, or do you remain resentful? What do you fight about, and why? It's important to know the answers to these questions, since startups move quickly and also go through tough times, making it likely that there will be disagreements and fights between the cofounders. If you don't know how you will be able to resolve these disagreements, you're setting yourself up for risk of failure.
One thing that Jessica advised is for you to actual work together with your cofounder for some time before signing up to build a company with them. Work on a side project with them, or maybe even spend a few months working full-time with them. This is similar to what I wrote about a few weeks ago: that it is really important for you to work on a project with your cofounder before starting a company with them. Before we started Dasient, my cofounders and I spent several months building Facebook apps together during nights and weekends. Working on a project together enables you to understand how complementary you are to each other, if each person takes on a fair share of responsibility, what each person's values/commitment level is, and finally how you resolve disagreements and differences. It also allows you to "kick the tires" and take your cofounder for a "test drive" before forming a company together.
I think Jessica's advice is spot on. Make sure you develop some experience actually working with your cofounders on a project so that you know how you fight (and move on) -- because there will be plenty of opportunities to disagree as you navigate the ups and downs of a startup.