I'll start with what was said about the way Robert De Niro is known to observe human behavior. It was said that actors who do that must be S's.
I don't think that's true at all. I don't think observing human behavior and using it in one's acting technique in any way points to someone being an S. In fact, it may even be an indicator of them being an N. I know this because I've acted before and done the exact same thing.
The thing about me is that I'm like a sponge- I tend to soak people in. Whenever I spend a lot of time around someone, I start acting like them. I pick up their voice inflections, mannerisms, accent, etc. Why do people think it's an S thing? It's not about the details for me, because I hardly pay attention to all the little tics in the way a person talks and moves. My combination of N and F just absorbs all of that in a subconscious way the same way I tend to absorb other people's emotional states. I'm just really good at spending time around someone, detecting their 'aura', and then imitating them effortlessly without thinking about it. It's something I've noticed other NF actors I've met can do very well, too.
Leaving that behind, though- His acting style is a known thing. And through what's known about it, I don't think it's S-derived at all. Robert De Niro was trained by Stella Adler, and it was a training that consisted of him learning how to act through his imagination. Stella Adler's acting technique was all about developing the imagination and then using it to immerse oneself in the imaginary circumstances of a play. In other words, Stella Adler-trained actors acted by turning their characters and scenes into personal fantasy-lives. Robert De Niro was known to be very loyal to this acting technique. And it's not S at all. Stella Adler once said that her acting style, the one Robert De Niro used, was empathy. The imagination comes from N and F. The fact that Robert De Niro was so good at this acting style lends me to believe that he was undoubtedly an NF, and quite possibly, as the OP stated, an INFJ.