My advice is to go with friends you already know. Don't "interview" for the position or "search" for people who may turn out to be something quite different than what you expected. A startup is a pressure cooker. If your personalities don't work together, it will end in an explosion.
There are as many problems as there are people but they generally come down to:
There is probably a whole host more but these are the ones I have seen others come unstuck with.
My advice is, if your looking for someone, friend or new, you need to cover off these points in a 1 to 2 page "in principal" and "heads of agreement" before taking it to a lawyer to become formalized. If you can't make it past agreeing to these points, your not suitable for each other.
There is probably a few others (happy to accept contributions).
Actually, you should ask yourself why you need a co-founder? if you can start something up alone, why all of the sudden needs a co-founder?
if come to a point that you meet a person, you want him to join your venture, and he has the value that you want. then it is crystal clear of what you expect.
if you just want somebody join you and share your workload, then it is better to hire somebody to work for you rather than give half of your company to him.