That used to be the case. These days, we have high-definition video monitors so that the actors really can communicate live over the screen (well, unless they're on redresses of the same set like Shatner and Montalban were). Heck, they had actors communicating live over TV screens as far back as Space: 1999, Battlestar Galactica, and Buck Rogers in the '70s, and even Doctor Who in the '60s -- it just didn't look as good as it does today. Star Trek didn't do it because its producers didn't think 20th-century cathode ray tubes would be convincing as futuristic communication devices, although a lot of other shows didn't mind. These days, though, HDTV screens have feature-quality resolution and are often used as practical viewscreens in SF productions. It's not stupid. The reason DS9 dropped it so quickly is because it was difficult to convey the difference between a person who was physically there and a holographic image, except by having the "hologram" stay motionless inside the projector frame, which was too static and visually uninteresting. And they are still getting ahead, because it's not about saving money. It's about allowing the actors to play off each other directly and give a better joint performance, and it's about freeing the directors to use a wider range of camera angles than they'd have available for shooting a viewscreen conversation.
There have been many instances in Trek history where cloaking devices have been "new," or where a breakthrough in cloaking penetration has seemed to disappear later on. The Suliban, Xyrillians, and Romulans had cloaks in Enterprise. Klingon cloaks could be spotted by visual distortion in TSFS but not in TUC. Spock found a way to detect them by engine emissions in TUC, but that was lost by TNG. And then there are the Mirror Klingons, who had cloaks in "Crossover" but not in "The Emperor's New Cloak." The logical explanation for all of it is that cloaking is not a single technology, but multiple different ones. There's an ongoing arms race between stealth and detection, and each time a way is invented to penetrate a given type of cloak, it becomes obsolete and cloaking has to be "invented" all over again.
I've always thought it would be a lot better in TWoK kind of situations to at least have the other actor on set to read the lines, but I guess the producers probably wouldn't want to pay the actors to be on set if they weren't actually on screen.
It wasn't about that. Ricardo Montalban was starring on Fantasy Island at the time, so he wasn't available to be on set at the same time as the main cast. His material was shot several months later than the Enterprise bridge scenes. I'd imagine that's the reason they didn't write a face-to-face Kirk/Khan meeting into the script -- because they knew the actors' schedules wouldn't allow it.
It's a case-by-case thing as to whether the actual actor is there or just a crewmember reading off screen (honestly, it often is for scenes where both characters are in the same room, with the actor who isn't in view or who has their back to the camera taking a break while the other person is shot in close-up. I've seen plenty of interviews where an actor says how great someone else to was to work with, and one of the examples was they always showed up for other people's close up where they were out of frame). As a specific example; Spoiler: The Last Jedi they actually flew Adam Driver out to the middle of nowhere, Ireland, so he could read opposite Daisy Ridley during their mental communications. I'm not sure whether the one scene where he actually was on the island, inside the hut, would've been shot on location or on a soundstage. Probably a soundstage, those huts were tiny and would be hard to shoot in if you couldn't remove walls and stuff, and they probably wouldn't have mentioned it was anything special if they were bringing him out on location anyway for that scene.
What interests me is when productions have actors turned invisible or transformed into motion-capture characters, or wearing CGI armor like the Iron Man suit, sometimes they'll still have the actual actor performing the part on set for the sake of the performance and playing off the other actors, even though it'd be simple enough to double it.
I'm pretty sure a big reason for it is that it's just more fun to shoot. Gimbaled sets that actually shake and rock are a blast for the actors (and catnip for making-of promotional shorts), even though with the camera shaking, too, you can't really tell the difference on-screen between the actors miming it or the real thing. I'm not sure how often Robert Downey Jr is in the full Iron Man suit with helmet (for obvious reasons), but I wouldn't be surprised if it was more than 0% of the time, if only because where's the fun in playing a super hero if you let your stunt-double do all the heroing? I suspect that's part of the reason why they've been building more complete Iron Man costumes over the years, too, even though they're still replacing it all with CG just like they did when RDJ was just wearing a plain black bodysuit for the first movie. Sure, it's helpful for the VFX to have one there to match when they put in the final version, but I'm not sure that balances out how cumbersome the suits must be to deal with. But actually having a fairly realistic Iron Man on set to react to and/or be inside? Actors and directors eat that stuff up. There is one I'm sure of when it was for fun, which is when Hayden Christensen insisted on being in the Darth Vader suit for the end of "Revenge of the Sith," even though he didn't really have the build for it. They had to put lifts in the boots so he'd match David Prowse and the other prior performers.
I think I remember hearing somewhere that Christensen begged them to let him being inside the suit. Some of the stuff set on Ach To was filmed on a mock-up of the location on Skellig Micheal. I remember some spy photos of recreations of the the huts and stuff around them somewhere off the island.
When they were not filming at Skellig Michael they were somewhere in the south of Ireland that looked similar
To me that just seems like a waste of money to fly him all the way there. I doubt the affect of acting can’t be that much better. Didn’t really think Daisy was all that great to be honest.
Were a lot of the Discovery scripts changed quite significantly from the early drafts to what eventually aired?
Please, I'm waiting for the reconciliation between Kobayashi Maru (Starfleet downgrades to primitive TOS technology to evade Romulan telepresence hacking) and Kelvin/Discovery level tech. Followed by another technological downgrade in time for TOS!
Oh, I disposed of that in my first Rise of the Federation book. The "downgrade" project proved unnecessary once the war ended and was abandoned, aside from some of its superficial aesthetics having an influence on future designs. I mean, really, I've always found it odd that people interpreted that passage to be an explanation for why Starfleet tech appeared simpler in TOS. Even if they did downgrade during the war, why in the world would anyone expect them to remain downgraded for a century after the war ended and the need ceased? That makes no sense. I'm not entirely sure what the authors of the Romulan War books intended by introducing that project, but it just doesn't seem reasonable for it to be that. After all, it's really rather silly to assume that the way a technology looks would really have anything to do with how advanced it is. Even aside from the fact that the difference is only in the technology levels of 1960s TV producers vs. later TV producers. That shouldn't be mistaken for a difference in in-universe advancement, any more than the recasting of Saavik should be mistaken for Saavik getting cosmetic surgery. What we see is an interpretation of the underlying reality, and sometimes it's only the interpretation that changes.