Yes-And attitude
I learnt the concept of Yes-And at Improve. It means, you are up for anything and will go along with whatever comes your way. It is necessary to take any scene forward. Without it you are more likely to be stuck in a limbo and ruin the scene. Your scene partner needs to trust your ability to Yes-And them and build on top of whichever direction they are taking.
I now look for a “yes-and” attitude in early employees. Ambiguity and stressful situations demand an optimistic person, who is willing to “yes-and” your ideas and thoughts, in spite of all the risks. A startup is doing 10 things bad, but one thing right. Ask yourself whether the person you are onboarding will celebrate that one thing, and maybe even find one more thing we can be good at. Or complain about the 10 things that are bad.
How to test for it? Work with them for a couple of days and put them in difficult situations — see how they react. If not, find it out using reference checks.