Si: Let’s just put it this way. Spock has the entire starfleet rulebook memorised. And fear not, he’ll inform you if you violate even the most minute aspect of these rules. It doesn’t matter how strong your friendship with Spock is; he will still cop at you if you break rules (not to say he isn’t extremely loyal). Details are embedded in Spock’s brain like women are in Kirk’s. Image is important to Spock, enough so that he he pretends not to assign Uhura to the Enterprise in order to “avoid the appearance of favouritism.” Spock has a difficult time believing anything that he hasn’t witnessed himself. Although he’s a problem solver when he needs to be, Spock’s primary talent is knowing and relaying the facts.
Te: Spock is also into logic. If it’s not logical, it’s wrong. He conforms to Sherlock Holmes’ philosophy “if you eliminated the impossible, whatever remains, however improbable must be the truth.” Spock shows his feelings by his actions, rather than his expression, dolling out punishment where he sees fit and acting to benefit people without necessarily explaining himself (thus the disagreement with Uhura). He’s a fairly decent diplomat and has little issue with conflict situations (unless they involve emotional manipulation from Kirk –those…have bad results). Spock knows how to find quick solutions to problems and has little difficulty carrying them out.
Fi: Spock doesn’t like his feelings, though he certainly understands the value of them. On the outside, Spock can often seem like a robot, but he’s actually highly emotional (beneath the surface). He’s reluctant to show them, or even feel them himself, and fights them with every fibre of his being. He has a strong ethical code that he lives to the letter…and often expects others to live as well. Even though Spock places strong importance on the traditions and values of his people, he’s also a bit of a non-conformist. He prefers to follow his own path, even if that path leads him away from everything familiar and comfortable.
Ne: Spock is really good at catastrophising. When in a dangerous fix, his mind immediately jumps to the worst possible outcome and assumes it as fact. “We will not fit.” Apparently, emotional compromisation is the end of the world, and there is only one way to do most things. When Spock is comfortable, however, he has no trouble coming up with some highly original strategies for dealing with the problems before him.
Yes, Leanard Nimoy’s Spock is ISTJ too.