Most partnership due diligence assesses capability. Can they do the work, do they have the relationships, have they done it before. All reasonable questions, and all secondary.
I have had three projects die within sight of completion. One was a product launch with a hundred thousand dollars of presales, killed when a partner quietly moved the promotional budget mid-campaign. One was a finished product with worldwide distribution secured and a quarter of a million in development behind it, destroyed by an argument over a five-thousand-dollar advance. One was a launch with fifty journalists in the room, and the partners simply did not turn up.
None of those failures were rational. Each one cost the person doing it more than it gained them.
People act from inside their own weather system
The thing I eventually understood is that everybody is acting reasonably from inside their own view of the situation. The problem is what has distorted that view — greed, ego, ambition, fear, self-righteousness. From inside, the five thousand dollars felt urgent. From outside, it was self-immolation.
You cannot argue somebody out of their own bubble system. You can only decide in advance whether to get into a boat with them.
What to actually look at
Time. How they treat yours. Chronic lateness is not a scheduling problem; it is a statement about whose hours count.
Exaggeration. Not lying — most people do not lie outright in a first meeting. Watch for the number that gets rounded generously, the client relationship described as closer than it is. Small inflations under no pressure predict large ones under pressure.
How they treat people who cannot help them. Family, junior staff, the person bringing the coffee. This is the single most reliable signal I know, and it costs nothing to observe.
Principles, asked about directly. Ask what they would not do, and why. People who have thought about it answer immediately. People who have not will improvise, and you will hear it.
Principles before positions
Victor Hugo has a line I keep coming back to: change your opinions, keep to your principles; change your leaves, keep your roots intact. Eleanor Roosevelt said something similar about Franklin — that he could navigate a crisis because the principles were settled long before it arrived, so the crisis only ever presented him with tactical questions.
That is what you are really testing for. Not whether somebody is nice. Whether they decided who they were before the pressure showed up.
Character first. Talent and connections after. I wouldn’t have written that sentence at thirty, and it’s cost me three times since to be sure of it.