These are great threads to have, but I do worry some underestimate the amount of interviewing they should do for a co-founder they've just met from here. As a rule of thumb if you can't imagine yourself being close friends with this person, it's probably not a good idea to have them as a co-founder. It's a much more important, long-term, and intimate relationship than a simple partnership or internship is. I'd suggest hanging out with prospective co-founders exhaustively, including plenty of idle, personal, and off-topic conversations before committing, and to remember that just like dating, there's a decent chance it may not be the perfect fit, so you have to be willing to move on and try again if so.