Give new people a season, not a verdict. Time runs the background check for you.

First impressions are fast and confident, and they are still guesses. Somebody quiet on day one may turn out to be the steadiest person in the group, and somebody magnetic in week one may be exhausting by week ten. You cannot shortcut this by being a good judge of character. You can only watch the same person across enough ordinary days for a pattern to appear.

Hold your opinion of new people loosely for a few months. Hand out neither the big trust nor the big write-off early. Pay attention to how they behave when nothing much is happening, because that is the version you get most of the time.