The most important element, in my opinion, is one that single questions alone cannot discern: Is this person a workplace psychopath?<p>I'm not kidding, and this is a real issue. A lot of people who can be great entrepreneurs can also have detrimental effects on your culture, the trust of your clients, and your long-term sustainability. Drive, initiative, passion, aptitude - these are qualities present in great founders and appear to be present in workplace psychopaths.<p>Get to know this person, and get a strong sense of their values - not from what they say and want you to believe, but from taking in the context of their accounts of what has worked out, and what is not, in their personal and professional life. For people they speak dismissively or negatively of, find them and get their side of the story. You cannot do too much due diligence here - this is critical, especially if you start to become widely successful. Then you're locked in with your partner in the co-pilot seat, and you won't have the desire to bolt or ability to change your decision without significant if not disastrous consequences to your business.<p>Your values must align, and you must trust this person with your life. Your startup will be your life for the next 7-10 years, by your planning.