What's funny is that I have not really thought about SJ in terms of a full MBTI type. But if I didn't look at anything else, I can see your reasoning:
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I: She's self-contained -- inner core that then interacts with the outer world, and back to the self.
S: Very down-to-earth, NOT ethereal. Practical, pragmatic, no-nonsense, earthy.
T: Fairly flat emotional delivery. Seems to process through the logic of something rather than emotionally reacting to it. Easily one of the most "emotionally flat" starlets [i.e., women who are commonly lusted after by the average man] I have ever seen.
P: Roll with the punches, easy-going, doesn't seem to fear much or need to be in control of things, can handle anything that comes her way. Rugged.
So if I did treat the function pairs separately (rather than reviewing the 16 MBTI archetypes), I would think ISTP.
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I cannot YouTube here, but I did read the interview:
1. I perceived her comments about acting strategy to be typical of Ti -- she perceives things in terms of figuring out what the film needs (logically), IOW, the core goals/essence of the film, then mentally working through it and determining what she has to deliver to fit that goal. She had many issues with people who carelessly insert themselves over the film (example: the girls who constantly wear the "sexy cute" face.)
2. She speaks with a lot of candor and doesn't seem apologetic for her words, whether it's how she's critiquing other people's efforts or using vulgar language casually. She doesn't go out of her way to offend, however (like an F who is trying to be portray themselves as anti-establishment would), it just feels like she is casual and "being her."
Comparing her to the ISTP I know, there are a few differences (because the ISTP is the eldest daughter of a pastor) such as the vulgarity... but in terms of ideas, both of them are not afraid to state what they perceive as core true, nor do they apologize for it. Some ideas are considered offensive, but they shrug off any of those feelings and are rather indifferent to reactions.
They also analyze well, are hands-on, pragmatic, feminine but not in the soft lacey / elegant way, just more "real" and "down to earth" about things. I really did enjoy talking with this girl, because she had such a good thinking process nor had many preconceptions; she was just observing, experiencing, and drawing conclusions from those things, and could laugh easily at human quirks and juxtapositions.
They feel sort of like "friend/cohort" material for me, to be honest -- people who see the world similarly to me in many ways, except I am much more whimsical and fanciful, and they're more rooted in what is actually happening (pragmatic).