From my experience, I would recommend finding someone with whom you are friendly, but not friends. The pressure of the startup will polarize your relationship,

From my experience, I would recommend finding someone with whom you are friendly, but not friends. The pressure of the startup will polarize your relationship, so you'll either end up pretty solid friends or pretty solid enemies. Someone whom you already dislike somewhat probably won't become any more palatable. Someone with whom you've already established a friendship leaves you in the position of risking the friendship and/or not having the luxury of candor when they screw up.