Vetting a co-founder can take a long time.
Some things I would want to find out:
"does this person have the right complementary skills?"
"Does this person have the mental strength and conviction to stick it out for 2 years?"
"Will we work well together?"
"What will they be like when things don't go as planned?"
Personalities and Incentives Why do you want to build a startup (in general and in particular, right now)? What motivates you (e.g. a technical challenge, an overall problem, helping the world, etc.) What do you do with your free time? (e.g. how do you unwind? How do you deal with stress and big challenges? What do you think I’d be most surprised to find out about you? What are our respective personal goals for the startup (e.g. a sustainable business that is spinning off cash and running it forever or high growth and some type of liquidity event)? Will this be the primary activity for each of us? What is the expected time commitment (now, in 6 months, 2 years, etc.)? Are we allowed to take on anything outside of the company? Would we sell this for $5mm? $100mm? Are we waiting for the billion dollar exit? Is there a part of our plan that we are unwilling to change (e.g. the product being built, the market being addressed or some other aspect of the company)? Pro-tip: the only guarantee is that things will change, so if someone isn’t open to that there is a problem. Personal Priorities What are our personal cash needs in the short term? Are we both going unpaid and when would this change? If you’re not able to go full-time now, under what circumstances would you be able to start working on something full-time? Do we want to raise outside money? If so, how much – now, in 6 months, 2 years, over the life of the company? Where do we want the business to be located? Get a sense of personal commitments that the person has so as to better manage your time and their time (i.e. are you married? With/without kids?) Have you ever failed at anything? If so, how did you handle it and what did you learn? If not >> big red flag! Talk about times you both did something you had never done before – how did it get handled and what was the outcome (could be technical or non-technical (e.g. I ran the internationalization of a site; I scaled a site to 5M users)? The Spouse Test- given how much time you’re going to spend with a cofounder, if you have a spouse or significant other you should have them meet the potential cofounder. Not only do they know you better than almost anyone and can give you a second opinion, but also they should know the person you’re going to spend a lot of time with (h/t to Bobber Interactive for this one). Working Styles and Culture If you had to come up with 3 words to describe the culture you want to create, what would they be (e.g. open, hard-working, eccentric)? Pro-tip: if you’re really serious and far along go visit few office spaces together to get a sense of what each of you likes work environments wise and why. What values do we want to instill in our employees? If you could pick 2 things to change and two things to bring with you from your previous companies, what would they be and why? How much equity are we allocating for future employees? Who do we think the first 5 hires will be?
A proper interview of a potential co-founder shouldn't be an interview. Get to know him/her, you will be on the same platform moving forwards.