[Aluminum Frost]

And what's the differences between how they interact?

 

[star tripper]

I think people figure out the last instinct... instinctively. Intuitively. If I were to describe the vibe of each type, I guess I'd say:

sx-last - a bit more impersonal. Not like professional, but like everything is at somewhat of a distance.
so-last - not visibly affected by the greater world
sp-last - focus on doing rather than being (throwing themselves into the world with reckless abandon relative to their type)

Kind of generic but it's a mostly reliable shorthand. I notice people often figure this out immediately, then they get sucked into enneagram and start analyzing and change to something else only to realize the initial "vibe" was correct all along.

 

[Dare]

I look at what they're bad at or deficient in or could go without (but beware those with a 'hidden' dominant type, where their dominant type looks like their blindspot at first glance -- e.g., the SO type that lives alone in a forrest writing manifestos).

If that isn't obvious, you can always rule out the one they fixate on (their dominant type) and then, between the two that are left, which is the one they're relatively better/'normal' at -- that will be their secondary. The one left is their blind spot.

I also suspect it's easy/intuitive to spot who is the same dominant type, as well as who is blind in your dominant type, based on 'vibe' -- that's how it is for me anyway.

 

[Mude]

I agree with both star tripper and Dare.

I would expect SP lasts to neglect or undervalue their security, comfort, mental/emotional integrity or physical health.
I would expect SO lasts to neglect or undervalue their responsibilities, reputation, networks or contributions to something more than themselves.
I would expect SX lasts to neglect or undervalue their passions, close relationships, creative energy, sources of inspiration, and personal fulfillment/satisfaction.

On interactions, I don't know if there's an easy answer. Stereotypes such as SPs being withdrawn or SOs being extroverted or SXs being moody just aren't consistent or reliable enough to judge by alone. Only talking to people and paying attention to what preoccupies them can really give a clue. That said, I think same "non-blinds" are likely to gravitate to each other. In other words, SO (or SP or SX) primary and secondaries are more likely to cross paths due to shared interests.

(but beware those with a 'hidden' dominant type, where their dominant type looks like their blindspot at first glance -- e.g., the SO type that lives alone in a forrest writing manifestos).

Who told you about me?