In the Shatner vein, my favorite didn't-notice-it-for-years acting moment was in The Naked Time, when Sulu was on the bridge with the sword. Specifically, the "Put -- put that thing away!" shot of Shatner. Watch it back in slow motion as Kirk puts his hand on the sword tip. His entire facial demeanor, in about a half second of real time, completely changes as he apparently realizes the potential threat level of the situation (not sure how else to describe it). It's mostly in the eyes...but it is amazing how he could manipulate his expression so subtly and so quickly!![]()
I think the secret of good acting is that he didn't manipulate his facial expression. He was "in the moment" and genuinely felt what Kirk was supposed to feel.
OTOH, I often thought "Sisko" on DS9 was deliberately contriving his facial expressions. He may have been a natural on Spenser (I didn't watch that show), but I think DS9 needed a lead with better training.