Therin, was it just the TNG movies that fell victim to that overzealous scrutiny and script leaks?
During TMP's production, Gene Roddenberry's Star Trek Office fed the curiosity of ST fans through regular memos to the fan clubs, personally signed by GR, regular newsletters from Majel Barrett's Lincoln Enterprises, and Susan Sackett's monthly columns in "Starlog". We were told that "In Thy Image" was the script being used to create the movie, and that it was based on "Robot's Return" (a "Genesis II" script treatment). these scripts were eventually available from Lincoln, but I'm sure that wasn't until after TMP came out. There were lots of pics officially released to the media before the premiere, including shots of the rusted Voyager space probe at the end!
For ST II, the big expose was Susan Sackett, working under instruction from the deposed GR himself(!), when she announced the (supposedly top secret) plans for Spock's death at a big UK convention. Nick Meyer and Harve Bennett thwarted the blown plot detail by adding a
faux Spock death in the
Kobayashi Maru scenario at the front, and then moving the real death to the end. Originally, Spock would have died about the time of Peter Preston. Harve Bennett later said in "Starlog" that Susan unwittingly improved the film by forcing them to reshuffle events. Pics were extremely rare. A few tiny ones of the new uniforms (and Saavik!) in Japanese "Super Visual" magazine and then Starlog's "Official Movie Magazine" a week or so before the US premiere.
With ST III, a bootleg copy of Harve Bennett's "Return to Genesis" script proposal was mass-produced - and sold at huge prices at big ST conventions in the US. It had Spock as a vampiric figure appearing in people's mirrors on the Enterprise! And a crew of Romulans on their Bird of Prey. Luckily, a lot was changed before the final version, which was kept pretty secret until the premiere. (The most raw footage we saw was a few months earlier, when the sets caught alight and Shatner posed for camera with a fire extinguisher.)
I was very lucky to buy some auctioned ST III call sheets in New York (a Creation convention) in January 1984, long before the US premiere! They revealed that an actor called "Frank Force" was playing a character called "Nacluv". The character's number was quite high, meaning he was introduced very late in the movie. (Of course, someone I knew from the set pointed out that "Nacluv" was "Vulcan" spelt backwards, and he was played by "the frankest force in the galaxy".) In the final credits, note that "Frank Force" is the Excelsior elevator voice! I also had morning tea with Grace Lee Whitney in LA a few weeks later and I accidentally found out she'd just done her Spacedock cafeteria cameo, filmed at ILM!
ST IV was also pretty secretive. Copying the ST III bootleg idea, a guy called John Flynn (who'd circulated a "Star Wars" script he'd tried submitting to Lucas) had written a phony proposal called "The Trial of James T Kirk", and it sold very well, but it was an obvious phony - from the sheer number of TOS guest artists it required.
There were many rumours of "whales and dolphins" being in the real film - and we knew that Eddie Murphy's role as a journalist type had been rewritten for a female marine biologist. The most intriguing "news" was "Entertainment Tonight" featuring the exciting filming of the Klingon vessel in Paramount's flooded carpark/"B" Tank, where Esther Williams once played a mermaid! My penpal drew me storyboards of everything she saw on her TV. (We eventually got the footage in Oz as "Entertainment This Week".) So we knew the crew went back in time, but most of us assumed the ship crashed into the bay when arriving in the 20th century! We couldn't think of how they would ever get back, esp. not with whales aboard!
There was definitely enough "news" to put a "sealed section" in my clubs newsletter!
With ST V, Roddenberry again got into a public sparring match, saying that Sybok was a bad idea. But again, there were not really any majorly leaked scripts. by this time, all scripts were hand numbered - in huge numerals through every page!
With ST VI, we knew there was a link with "Unification", and eventually someone put together several sets of numbered bootlegged pages to create a whole script - with Saavik's name in the place of Valeris. But it didn't get wide release until the movie premiered.