[PeterisP]

In essence, will the existing team like working together with that person, ignoring all the purely professional aspects. Personality, communication, agreeability, compatibility (or conflict-avoidance) regarding various non-work opinions, alignment regarding subjective styles of work (e.g. 'move fast and break things' vs 'verify everything 3 times, even if it causes a delay' - you need to fit the organization's "style"), etc.

[almostdeadguy]

Frankly an organization's "style" is a bunch of unexamined, calcified social biases that companies screen in interviews for because they don't like to be challenged on them. Or in the more malevolent cases its outright bigotry that's given cover as something else. Heterogeneity of methodology is actually good (especially at startups, which often operate in ways purely driven by the whims of inexperienced founders), and the people you interview can often teach you a lot. "Culture" is a joke, there's nothing cultural about these things.

[slededit]

Microsoft in the 90s used to hire on talent and wall off anyone who "didn't get along". They could code in peace and their manager handled outside communication.